doe fundamentals handbook -...

DOE-HDBK-1018/1-93JANUARY 1993

DOE FUNDAMENTALS HANDBOOKMECHANICAL SCIENCEVolume 1 of 2

U.S. Department of Energy FSC-6910Washington, D.C. 20585

Distribution Statement A. Approved for public release; distribution is unlimited.

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This Portable Document Format (PDF) file contains bookmarks, thumbnails, and hyperlinks to help you navigate through the document. The modules listed in the Overview are linked to the corresponding pages. Text headings in each module are linked to and from the table of contents for that module. Click on the DOE seal below to move to the Overview.

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Available to DOE and DOE contractors from the Office of Scientific andTechnical Information. P.O. Box 62, Oak Ridge, TN 37831.

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Order No. DE93012178


DOE-HDBK-1018/1-93MECHANICAL SCIENCE

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ABSTRACT

The Mechanical Science Handbook was developed to assist nuclear facility operatingcontractors in providing operators, maintenance personnel, and the technical staff with the necessaryfundamentals training to ensure a basic understanding of mechanical components and mechanicalscience. The handbook includes information on diesel engines, heat exchangers, pumps, valves, andmiscellaneous mechanical components. This information will provide personnel with a foundationfor understanding the construction and operation of mechanical components that are associated withvarious DOE nuclear facility operations and maintenance.

Key Words: Training Material, Diesel Engine, Heat Exchangers, Pumps, Valves



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FOREWORD

The Department of Energy (DOE) Fundamentals Handbooks consist of ten academicsubjects, which include Mathematics; Classical Physics; Thermodynamics, Heat Transfer, and FluidFlow; Instrumentation and Control; Electrical Science; Material Science; Mechanical Science;Chemistry; Engineering Symbology, Prints, and Drawings; and Nuclear Physics and ReactorTheory. The handbooks are provided as an aid to DOE nuclear facility contractors.

These handbooks were first published as Reactor Operator Fundamentals Manuals in 1985for use by DOE category A reactors. The subject areas, subject matter content, and level of detailof the Reactor Operator Fundamentals Manuals were determined from several sources. DOECategory A reactor training managers determined which materials should be included, and servedas a primary reference in the initial development phase. Training guidelines from the commercialnuclear power industry, results of job and task analyses, and independent input from contractors andoperations-oriented personnel were all considered and included to some degree in developing thetext material and learning objectives.

The DOE Fundamentals Handbooks represent the needs of various DOE nuclear facilities'fundamental training requirements. To increase their applicability to nonreactor nuclear facilities,the Reactor Operator Fundamentals Manual learning objectives were distributed to the NuclearFacility Training Coordination Program Steering Committee for review and comment. To updatetheir reactor-specific content, DOE Category A reactor training managers also reviewed andcommented on the content. On the basis of feedback from these sources, information that appliedto two or more DOE nuclear facilities was considered generic and was included. The final draft ofeach of the handbooks was then reviewed by these two groups. This approach has resulted inrevised modular handbooks that contain sufficient detail such that each facility may adjust thecontent to fit their specific needs.

Each handbook contains an abstract, a foreword, an overview, learning objectives, and textmaterial, and is divided into modules so that content and order may be modified by individual DOEcontractors to suit their specific training needs. Each handbook is supported by a separateexamination bank with an answer key.

The DOE Fundamentals Handbooks have been prepared for the Assistant Secretary forNuclear Energy, Office of Nuclear Safety Policy and Standards, by the DOE Training CoordinationProgram. This program is managed by EG&G Idaho, Inc.



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OVERVIEW

The Department of Energy Fundamentals Handbook entitled Mechanical Science wasprepared as an information resource for personnel who are responsible for the operation of theDepartment's nuclear facilities. Almost all processes that take place in the nuclear facilities involvethe use of mechanical equipment and components. A basic understanding of mechanical science isnecessary for DOE nuclear facility operators, maintenance personnel, and the technical staff tosafely operate and maintain the facility and facility support systems. The information in thehandbook is presented to provide a foundation for applying engineering concepts to the job. Thisknowledge will help personnel more fully understand the impact that their actions may have on thesafe and reliable operation of facility components and systems.

The Mechanical Science handbook consists of five modules that are contained in twovolumes. The following is a brief description of the information presented in each module of thehandbook.

Volume 1 of 2

Module 1 - Diesel Engine Fundamentals

Provides information covering the basic operating principles of 2-cycle and 4-cyclediesel engines. Includes operation of engine governors, fuel ejectors, and typicalengine protective features.

Module 2 - Heat Exchangers

Describes the construction of plate heat exchangers and tube and shell heatexchangers. Describes the flow patterns and temperature profiles in parallel flow,counter flow, and cross flow heat exchangers.

Module 3 - Pumps

Explains the operation of centrifugal and positive displacement pumps. Topicsinclude net positive suction head, cavitation, gas binding, and pump characteristiccurves.



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OVERVIEW (Cont.)

Volume 2 of 2

Module 4 - Valves

Introduces the functions of the basic parts common to most types of valves.Provides information on applications of many types of valves. Types of valvescovered include gate valves, globe valves, ball valves, plug valves, diaphragmvalves, reducing valves, pinch valves, butterfly valves, needle valves, check valves,and safety/relief valves.

Module 5 - Miscellaneous Mechanical Components

Provides information on significant mechanical devices that have widespreadapplication in nuclear facilities but do not fit into the categories of componentscovered by the other modules. These include cooling towers, air compressors,demineralizers, filters, strainers, etc.

The information contained in this handbook is not all-encompassing. An attempt to presentthe entire subject of mechanical science would be impractical. However, the Mechanical Sciencehandbook presents enough information to provide the reader with the fundamental knowledgenecessary to understand the advanced theoretical concepts presented in other subject areas, and tounderstand basic system and equipment operation.


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Department of EnergyFundamentals Handbook

MECHANICAL SCIENCEModule 1

Diesel Engine Fundamentals


Diesel Engine Fundamentals DOE-HDBK-1018/1-93 TABLE OF CONTENTS

TABLE OF CONTENTS

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

DIESEL ENGINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Diesel Engines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Major Components of a Diesel Engine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Diesel Engine Support Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Exhaust System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Operational Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

FUNDAMENTALS OF THE DIESEL CYCLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

The Basic Diesel Cycles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21The Four-Stoke Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22The Two-Stroke Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

DIESEL ENGINE SPEED, FUEL CONTROLS,AND PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Engine Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Fuel Injectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Governor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Operation of a Governor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Starting Circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Engine Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

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LIST OF FIGURES DOE-HDBK-1018/1-93 Diesel Engine Fundamentals

LIST OF FIG URES

Figure 1 Example of a Large Skid-Mounted, Diesel-Driven Generator. . . . . . . . . . . . . . 2

Figure 2 Cutaway of a Four-Stroke Supercharged Diesel Engine . . . . . . . . . . . . . . . . . . 4

Figure 3 Cross Section of a V-type Four Stroke Diesel Engine. . . . . . . . . . . . . . . . . . . 5

Figure 4 The Cylinder Block. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 5 Diesel Engine Wet Cylinder Sleeve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6 Piston and Piston Rod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 7 Diesel Engine Crankshaft and Bearings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 8 Diesel Engine Valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 9 Diesel Engine Camshaft and Drive Gear. . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 10 Diesel Engine Valve Train. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 11 Diesel Engine Cooling System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 12 Diesel Engine Internal Lubrication System. . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 13 Diesel Engine Fuel Flowpath. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 14 Oil Bath Air Filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 15 Compression Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 16 Scavenging and Intake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 17 Compression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 18 Fuel Injection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 19 Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 20 Exhaust. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 21 2-Stroke Exhaust. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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Diesel Engine Fundamentals DOE-HDBK-1018/1-93 LIST OF FIGURES

LIST OF FIGURES (Cont.)

Figure 22 2-Stroke Intake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 23 2-Stroke Compression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 24 2-Stroke Fuel Injection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 25 2-Stroke Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 26 Fuel Injector Cutaway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 27 Fuel Injector Plunger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 28 Simplified Mechanical-Hydraulic Governor. . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 29 Cutaway of a Woodward Governor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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LIST OF TABLES DOE-HDBK-1018/1-93 Diesel Engine Fundamentals

LIST OF TABLES

NONE

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Diesel Engine Fundamentals DOE-HDBK-1018/1-93 REFERENCES

REFERENCES

Benson & Whitehouse, Internal Combustion Engines, Pergamon.

Cheremisinoff, N. P., Fluid Flow, Pumps, Pipes and Channels, Ann Arbor Science.

Scheel, Gas and Air Compression Machinery, McGraw/Hill.

Skrotzki and Vopat, Steam and Gas Turbines, McGraw/Hill.

Stinson, Karl W., Diesel Engineering Handbook, Diesel Publications Incorporated.

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OBJECTIVES DOE-HDBK-1018/1-93 Diesel Engine Fundamentals

TERMINAL OBJECTIVE

1.0 Without references, DESCRIBE the components and theory of operation for a dieselengine.

ENABLING OBJECTIVES

1.1 DEFINE the following diesel engine terms:

a. Compression ratiob. Borec. Stroked. Combustion chamber

1.2 Given a drawing of a diesel engine, IDENTIFY the following:

a. Piston/rodb. Cylinderc. Blowerd. Crankshaft

e. Intake ports or valve(s)f. Exhaust ports or valve(s)g. Fuel injector

1.3 EXPLAIN how a diesel engine converts the chemical energy stored in the diesel fuel intomechanical energy.

1.4 EXPLAIN how the ignition process occurs in a diesel engine.

1.5 EXPLAIN the operation of a 4-cycle diesel engine to include when the following eventsoccur during a cycle:

a. Intakeb. Exhaustc. Fuel injectiond. Compressione. Power

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Diesel Engine Fundamentals DOE-HDBK-1018/1-93 OBJECTIVES

ENABLING OBJECTIVES (Cont.)

1.6 EXPLAIN the operation of a 2-cycle diesel engine, including when the following eventsoccur during a cycle:


1.7 DESCRIBE how the mechanical-hydraulic governor on a diesel engine controls enginespeed.

1.8 LIST five protective alarms usually found on mid-sized and larger diesel engines.

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OBJECTIVES DOE-HDBK-1018/1-93 Diesel Engine Fundamentals

Intentionally Left Blank

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Diesel Engine Fundamentals DOE-HDBK-1018/1-93 DIESEL ENGINES

DIESEL ENGINES

One of the most common prime movers is the diesel engine. Before gaining anunderstanding of how the engine operates a basic understanding of the engine'scomponents must be gained. This chapter reviews the major components of ageneric diesel engine.

EO 1.1 DEFINE the following diesel engine terms:

a. Compression ratiob. Borec. Stroked. Combustion chamber

EO 1.2 Given a drawing of a diesel engine, IDENTIFY the following:

a. Piston/rodb. Cylinderc. Blowerd. Crankshaft

e. Intake ports or valve(s)f. Exhaust ports or valve(s)g. Fuel injector

I ntr oduction

Most DOE facilities require some type of prime mover to supply mechanical power for pumping,electrical power generation, operation of heavy equipment, and to act as a backup electricalgenerator for emergency use during the loss of the normal power source. Although several typesof prime movers are available (gasoline engines, steam and gas turbines), the diesel engine isthe most commonly used. Diesel engines provide a self-reliant energy source that is availablein sizes from a few horsepower to 10,000 hp. Figure 1 provides an illustration of a commonskid-mounted, diesel-driven generator. Relatively speaking, diesel engines are small,inexpensive, powerful, fuel efficient, and extremely reliable if maintained properly.

Because of the widespread use of diesel engines at DOE facilities, a basic understanding of theoperation of a diesel engine will help ensure they are operated and maintained properly. Due tothe large variety of sizes, brands, and types of engines in service, this module is intended toprovide the fundamentals and theory of operation of a diesel engine. Specific information ona particular engine should be obtained from the vendor's manual.

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DIESEL ENGINES DOE-HDBK-1018/1-93 Diesel Engine Fundamentals

Histor y

Figure 1 Example of a Large Skid-Mounted, Diesel-Driven Generator

The modern diesel engine came about as the result of the internal combustion principles firstproposed by Sadi Carnot in the early 19th century. Dr. Rudolf Diesel applied Sadi Carnot'sprinciples into a patented cycle or method of combustion that has become known as the "diesel"cycle. His patented engine operated when the heat generated during the compression of the airfuel charge caused ignition of the mixture, which then expanded at a constant pressure duringthe full power stroke of the engine.

Dr. Diesel's first engine ran on coal dust and used a compression pressure of 1500 psi toincrease its theoretical efficiency. Also, his first engine did not have provisions for any type ofcooling system. Consequently, between the extreme pressure and the lack of cooling, the engineexploded and almost killed its inventor. After recovering from his injuries, Diesel tried againusing oil as the fuel, adding a cooling water jacket around the cylinder, and lowering thecompression pressure to approximately 550 psi. This combination eventually proved successful.Production rights to the engine were sold to Adolphus Bush, who built the first diesel enginesfor commercial use, installing them in his St. Louis brewery to drive various pumps.

Diesel Engines

A diesel engine is similar to the gasoline engine used in most cars. Both engines are internalcombustion engines, meaning they burn the fuel-air mixture within the cylinders. Both arereciprocating engines, being driven by pistons moving laterally in two directions. The majorityof their parts are similar. Although a diesel engine and gasoline engine operate with similarcomponents, a diesel engine, when compared to a gasoline engine of equal horsepower, isheavier due to stronger, heavier materials used to withstand the greater dynamic forces from thehigher combustion pressures present in the diesel engine.

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The greater combustion pressure is the result of the higher compression ratio used by dieselengines. The compression ratio is a measure of how much the engine compresses the gasses inthe engine's cylinder. In a gasoline engine the compression ratio (which controls thecompression temperature) is limited by the air-fuel mixture entering the cylinders. The lowerignition temperature of gasoline will cause it to ignite (burn) at a compression ratio of less than10:1. The average car has a 7:1 compression ratio. In a diesel engine, compression ratiosranging from 14:1 to as high as 24:1 are commonly used. The higher compression ratios arepossible because only air is compressed, and then the fuel is injected. This is one of the factorsthat allows the diesel engine to be so efficient. Compression ratio will be discussed in greaterdetail later in this module.

Another difference between a gasoline engine and a diesel engine is the manner in which enginespeed is controlled. In any engine, speed (or power) is a direct function of the amount of fuelburned in the cylinders. Gasoline engines are self-speed-limiting, due to the method the engineuses to control the amount of air entering the engine. Engine speed is indirectly controlled bythe butterfly valve in the carburetor. The butterfly valve in a carburetor limits the amount ofair entering the engine. In a carburetor, the rate of air flow dictates the amount of gasoline thatwill be mixed with the air. Limiting the amount of air entering the engine limits the amount offuel entering the engine, and, therefore, limits the speed of the engine. By limiting the amountof air entering the engine, adding more fuel does not increase engine speed beyond the pointwhere the fuel burns 100% of the available air (oxygen).

Diesel engines are not self-speed-limiting because the air (oxygen) entering the engine is alwaysthe maximum amount. Therefore, the engine speed is limited solely by the amount of fuelinjected into the engine cylinders. Therefore, the engine always has sufficient oxygen to burn andthe engine will attempt to accelerate to meet the new fuel injection rate. Because of this, amanual fuel control is not possible because these engines, in an unloaded condition, canaccelerate at a rate of more than 2000 revolutions per second. Diesel engines require a speedlimiter, commonly called the governor, to control the amount of fuel being injected into theengine.

Unlike a gasoline engine, a diesel engine does not require an ignition system because in a dieselengine the fuel is injected into the cylinder as the piston comes to the top of its compressionstroke. When fuel is injected, it vaporizes and ignites due to the heat created by thecompression of the air in the cylinder.

Majo r Components of a Diesel Engine

To understand how a diesel engine operates, an understanding of the major components and howthey work together is necessary. Figure 2 is an example of a medium-sized, four-stroke,supercharged, diesel engine with inlet ports and exhaust valves. Figure 3 provides a crosssection of a similarly sized V-type diesel engine.

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Figure 2 Cutaway of a GM V-16 Four-Stroke Supercharged Diesel Engine

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Figure 3 Cross Section of a V-type Four Stroke Diesel Engine

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The Cylinder Block

The cylinder block, as shown in Figure 4, is generally a single unit made from cast iron.In a liquid-cooled diesel, the block also provides the structure and rigid frame for theengine's cylinders, water coolant and oil passages, and support for the crankshaft andcamshaft bearings.

Figure 4 The Cylinder Block

Cr ankcase and Oil Pan

The crankcase is usually located on the bottom of the cylinder block. The crankcase isdefined as the area around the crankshaft and crankshaft bearings. This area encloses therotating crankshaft and crankshaft counter weights and directs returning oil into the oilpan. The oil pan is located at the bottom of the crankcase as shown in Figure 2 andFigure 3. The oil pan collects and stores the engine's supply of lubricating oil. Largediesel engines may have the oil pan divided into several separate pans.

Cylinder Sleeve or Bor e

Diesel engines use one of two types of cylinders. In one type, each cylinder is simplymachined or bored into the block casting, making the block and cylinders an integralpart. In the second type, a machined steel sleeve is pressed into the block casting to formthe cylinder. Figure 2 and Figure 3 provide examples of sleeved diesel engines. Witheither method, the cylinder sleeve or bore provides the engine with the cylindricalstructure needed to confine the combustion gasses and to act as a guide for the engine'spistons.

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In engines using sleeves, there are two

Figure 5 Diesel Engine Wet Cylinder Sleeve

types of sleeves, wet and dry. A drysleeve is surrounded by the metal ofthe block and does not come in directcontact with the engine's coolant(water). A wet sleeve comes in directcontact with the engine's coolant.Figure 5 provides an example of a wetsleeve. The volume enclosed by thesleeve or bore is called the combustionchamber and is the space where thefuel is burned.

In either type of cylinder, sleeved orbored, the diameter of the cylinder iscalled the bore of the engine and isstated in inches. For example, thebore of a 350 cubic inch Chevroletgasoline engine is 4 inches.

Most diesel engines are multi-cylinderengines and typically have theircylinders arranged in one of twoways, an in-line or a "V", although other combinations exits. In an in-line engine, as thename indicates, all the cylinders are in a row. In a "V" type engine the cylinders arearranged in two rows of cylinders set at an angle to each other that align to a commoncrankshaft. Each group of cylinders making up one side of the "V" is referred to as abank of cylinders.

Figure 6 Piston and Piston Rod

Piston and Piston Rings

The piston transforms the energy ofthe expanding gasses intomechanical energy. The piston ridesin the cylinder liner or sleeve asshown in Figure 2 and Figure 3.Pistons are commonly made ofaluminum or cast iron alloys.

To prevent the combustion gassesfrom bypassing the piston and tokeep friction to a minimum, eachpiston has several metal rings aroundit, as illustrated by Figure 6.

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These rings function as the seal between the piston and the cylinder wall and also act toreduce friction by minimizing the contact area between the piston and the cylinder wall.The rings are usually made of cast iron and coated with chrome or molybdenum. Mostdiesel engine pistons have several rings, usually 2 to 5, with each ring performing adistinct function. The top ring(s) acts primarily as the pressure seal. The intermediatering(s) acts as a wiper ring to remove and control the amount of oil film on the cylinderwalls. The bottom ring(s) is an oiler ring and ensures that a supply of lubricating oil isevenly deposited on the cylinder walls.

Connecting Rod

The connecting rod connects the piston to the crankshaft. See Figure 2 and Figure 3 forthe location of the connecting rods in an engine. The rods are made from drop-forged,heat-treated steel to provide the required strength. Each end of the rod is bored, with thesmaller top bore connecting to the piston pin (wrist pin) in the piston as shown inFigure 6. The large bore end of the rod is split in half and bolted to allow the rod to beattached to the crankshaft. Some diesel engine connecting rods are drilled down thecenter to allow oil to travel up from the crankshaft and into the piston pin and piston forlubrication.

A variation found in V-type engines that affects the connecting rods is to position thecylinders in the left and right banks directly opposite each other instead of staggered(most common configuration). This arrangement requires that the connecting rods of twoopposing cylinders share the same main journal bearing on the crankshaft. To allow thisconfiguration, one of the connecting rods must be split or forked around the other.

Cr ankshaft

The crankshaft transforms the linear motion of the pistons into a rotational motion thatis transmited to the load. Crankshafts are made of forged steel. The forged crankshaftis machined to produce the crankshaft bearing and connecting rod bearing surfaces. Therod bearings are eccentric, or offset, from the center of the crankshaft as illustrated inFigure 7. This offset converts the reciprocating (up and down) motion of the piston intothe rotary motion of the crankshaft. The amount of offset determines the stroke (distancethe piston travels) of the engine (discussed later).

The crankshaft does not ride directly on the cast iron block crankshaft supports, but rideson special bearing material as shown in Figure 7. The connecting rods also havebearings inserted between the crankshaft and the connecting rods. The bearing materialis a soft alloy of metals that provides a replaceable wear surface and prevents gallingbetween two similar metals (i.e., crankshaft and connecting rod). Each bearing is splitinto halves to allow assembly of the engine. The crankshaft is drilled with oil passagesthat allow the engine to feed oil to each of the crankshaft bearings and connection rodbearings and up into the connecting rod itself.

The crankshaft has large weights, called counter weights, that balance the weight of theconnecting rods. These weights ensure an even (balance) force during the rotation ofthe moving parts.

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Figure 7 Diesel Engine Crankshaft and Bearings

Flywheel

The flywheel is located on one end of the crankshaft and serves three purposes. First,through its inertia, it reduces vibration by smoothing out the power stroke as eachcylinder fires. Second, it is the mounting surface used to bolt the engine up to its load.Third, on some diesels, the flywheel has gear teeth around its perimeter that allow thestarting motors to engage and crank the diesel.

Cylinder Heads and Valves

A diesel engine's cylinder heads perform several functions. First, they provide the topseal for the cylinder bore or sleeve. Second, they provide the structure holding exhaustvalves (and intake valves where applicable), the fuel injector, and necessary linkages. Adiesel engine's heads are manufactured in one of two ways. In one method, eachcylinder has its own head casting, which is bolted to the block. This method is usedprimarily on the larger diesel engines. In the second method, which is used on smallerengines, the engine's head is cast as one piece (multi-cylinder head).

Diesel engines have two methods of admitting and exhausting gasses from the cylinder.They can use either ports or valves or a combination of both. Ports are slots in thecylinder walls located in the lower 1/3 of the bore. See Figure 2 and Figure 3 forexamples of intake ports, and note their relative location with respect to the rest of the

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engine. When the piston travels below the level of the ports, the ports are "opened" andfresh air or exhaust gasses are able to enter or leave, depending on the type of port.

The ports are then "closed" when the

Figure 8 Diesel Engine Valve

piston travels back above the level ofthe ports. Valves (refer to figure 8)are mechanically opened and closed toadmit or exhaust the gasses as needed.The valves are located in the headcasting of the engine. The point atwhich the valve seals against the headis called the valve seat. Mostmedium-sized diesels have eitherintake ports or exhaust valves or bothintake and exhaust valves.

T iming Gears, Camshaft, andValve Mechanism

In order for a diesel engine tooperate, all of its components mustperform their functions at very precise intervals in relation to the motion of the piston.To accomplish this, a component called a camshaft is used. Figure 9 illustrates acamshaft and camshaft drive gear. Figure 2 and Figure 3 illustrate the location of acamshaft in a large overhead cam diesel engine.

Figure 9 Diesel Engine Camshaft and Drive Gear

A camshaft is a longbar with egg-shapedeccentric lobes, onelobe for each valve andfuel injector (discussedlater). Each lobe has afollower as shown onFigure 10. As thecamshaft is rotated, thefollower is forced upand down as it followsthe profile of the camlobe. The followers areconnected to theengine's valves and fueli n j e c t o r s t h r o u g hvar ious types oflinkages called pushrodsand rocker arms. The

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pushrods and rocker arms transfer the reciprocating motion generated by the camshaftlobes to the valves and injectors, opening and closing them as needed. The valves aremaintained closed by springs.

As the valve is opened by the camshaft, it compresses the valve spring. The energystored in the valve spring is then used to close the valve as the camshaft lobe rotates outfrom under the follower. Because an engine experiences fairly large changes intemperature (e.g., ambient to a normal running temperature of about 190°F), itscomponents must be designed to allow for thermal expansion. Therefore, the valves,valve pushrods, and rocker arms must have some method of allowing for the expansion.This is accomplished by the use of valve lash. Valve lash is the term given to the "slop"or "give" in the valve train before the cam actually starts to open the valve.

The camshaft is driven by

Figure 10 Diesel Engine Valve Train

the engine's crankshaftthrough a series of gearscalled idler gears andtiming gears. The gearsallow the rotation of thecamshaft to correspond orbe in time with, therotation of the crankshaftand thereby allows thevalve opening, valveclosing, and injection offuel to be timed to occur atprecise intervals in thepiston's travel. Toincrease the flexibility intiming the valve opening,valve closing, and injectionof fuel, and to increasepower or to reduce cost,an engine may have one ormore camshafts. Typically,in a medium to large V-type engine, each bank will have one or more camshafts per head.In the larger engines, the intake valves, exhaust valves, and fuel injectors may share acommon camshaft or have independent camshafts.

Depending on the type and make of the engine, the location of the camshaft or shaftsvaries. The camshaft(s) in an in-line engine is usually found either in the head of theengine or in the top of the block running down one side of the cylinder bank. Figure 10provides an example of an engine with the camshaft located on the side of the engine.Figure 3 provides an example of an overhead cam arrangement as on a V-type engine.On small or mid-sized V-type engines, the camshaft is usually located in the block at the

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center of the "V" between the two banks of cylinders. In larger or multi-camshafted V-type engines, the camshafts are usually located in the heads.

Blower

The diesel engine's blower is part of the air intake system and serves to compress theincoming fresh air for delivery to the cylinders for combustion. The location of theblower is shown on Figure 2. The blower can be part of either a turbocharged orsupercharged air intake system. Additional information on these two types of blowers isprovided later in this module.

Diesel Engine Suppor t Systems

A diesel engine requires five supporting systems in order to operate: cooling, lubrication, fuelinjection, air intake, and exhaust. Depending on the size, power, and application of the diesel,these systems vary in size and complexity.

Engine Cooling

Figure 11 Diesel Engine Cooling System

Nearly all dieselengines rely on al iqu id coo l ingsystem to transferwaste heat out ofthe block andinternals as shownin Figure 11. Thecooling systemconsists of a closedloop similar to thatof a car engine andc o n t a i n s t h efollowing majorcomponents: waterpump, radiator orheat exchanger,water jacket (whichconsists of coolantpassages in theblock and heads),and a thermostat.

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Engine L ubr ication

An internal combustion engine would not run for even a few minutes if the moving partswere allowed to make metal-to-metal contact. The heat generated due to the tremendousamounts of friction would melt the metals, leading to the destruction of the engine. Toprevent this, all moving parts ride on a thin film of oil that is pumped between all themoving parts of the engine.

Once between the moving parts, the oil serves two purposes. One purpose is to lubricatethe bearing surfaces. The other purpose is to cool the bearings by absorbing the friction-generated heat. The flow of oil to the moving parts is accomplished by the engine'sinternal lubricating system.

Figure 12 Diesel Engine Internal Lubrication System

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Oil is accumulated and stored in the engine's oil pan where one or more oil pumps takea suction and pump the oil through one or more oil filters as shown in Figure 12. Thefilters clean the oil and remove any metal that the oil has picked up due to wear. Thecleaned oil then flows up into the engine's oil galleries. A pressure relief valve(s)maintains oil pressure in the galleries and returns oil to the oil pan upon high pressure.The oil galleries distribute the oil to all the bearing surfaces in the engine.

Once the oil has cooled and lubricated the bearing surfaces, it flows out of the bearingand gravity-flows back into the oil pan. In medium to large diesel engines, the oil is alsocooled before being distributed into the block. This is accomplished by either an internalor external oil cooler. The lubrication system also supplies oil to the engine's governor,which is discussed later in this module.

Fuel System

All diesel engines require a method to store and deliver fuel to the engine. Becausediesel engines rely on injectors which are precision components with extremely tighttolerances and very small injection hole(s), the fuel delivered to the engine must beextremely clean and free of contaminants.

The fuel system must, therefore,

Figure 13 Diesel Engine Fuel Flowpath

not only deliver the fuel but alsoensure its cleanliness. This isusually accomplished through aseries of in-l ine f i l ters.Commonly, the fuel will befiltered once outside the engineand then the fuel will pass throughat least one more filter internal tothe engine, usually located in thefuel line at each fuel injector.

In a diesel engine, the fuel systemis much more complex than thefuel system on a simple gasolineengine because the fuel serves twopurposes. One purpose isobviously to supply the fuel to run the engine; the other is to act as a coolant to theinjectors. To meet this second purpose, diesel fuel is kept continuously flowing throughthe engine's fuel system at a flow rate much higher than required to simply run theengine, an example of a fuel flowpath is shown in Figure 13. The excess fuel is routedback to the fuel pump or the fuel storage tank depending on the application.

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Air I ntake System

Because a diesel engine requires close tolerances to achieve its compression ratio, andbecause most diesel engines are either turbocharged or supercharged, the air entering theengine must be clean, free of debris, and as cool as possible. Turbocharging andsupercharging are discussed in more detail later in this chapter. Also, to improve aturbocharged or supercharged engine's efficiency, the compressed air must be cooled afterbeing compressed. The air intake system is designed to perform these tasks.

Air intake systems vary greatly

Figure 14 Oil Bath Air Filter

from vendor to vendor but areusually one of two types, wet ordry. In a wet filter intake system,as shown in Figure 14, the air issucked or bubbled through ahousing that holds a bath of oilsuch that the dirt in the air isremoved by the oil in the filter.The air then flows through ascreen-type material to ensure anyentrained oil is removed from theair. In a dry filter system, paper,cloth, or a metal screen material isused to catch and trap dirt beforeit enters the engine (similar to thetype used in automobile engines).

In addition to cleaning the air, theintake system is usually designedto intake fresh air from as faraway from the engine aspracticable, usually just outside ofthe engine's building or enclosure.This provides the engine with asupply of air that has not beenheated by the engine's own wasteheat.

The reason for ensuring that an engine's air supply is as cool as possible is that cool airis more dense than hot air. This means that, per unit volume, cool air has more oxygenthan hot air. Thus, cool air provides more oxygen per cylinder charge than less dense,hot air. More oxygen means a more efficient fuel burn and more power.

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After being filtered, the air is routed by the intake system into the engine's intakemanifold or air box. The manifold or air box is the component that directs the fresh airto each of the engine's intake valves or ports. If the engine is turbocharged orsupercharged, the fresh air will be compressed with a blower and possibly cooled beforeentering the intake manifold or air box. The intake system also serves to reduce the airflow noise.

Tur bochar ging

Turbocharging an engine occurs when the engine's own exhaust gasses are forcedthrough a turbine (impeller), which rotates and is connected to a second impellerlocated in the fresh air intake system. The impeller in the fresh air intake systemcompresses the fresh air. The compressed air serves two functions. First, itincreases the engine's available power by increasing the maximum amount of air(oxygen) that is forced into each cylinder. This allows more fuel to be injectedand more power to be produced by the engine. The second function is to increaseintake pressure. This improves the scavenging of the exhaust gasses out of thecylinder. Turbocharging is commonly found on high power four-stroke engines.It can also be used on two-stroke engines where the increase in intake pressuregenerated by the turbocharger is required to force the fresh air charge into thecylinder and help force the exhaust gasses out of the cylinder to enable the engineto run.

Super char ging

Supercharging an engine performs the same function as turbocharging an engine.The difference is the source of power used to drive the device that compresses theincoming fresh air. In a supercharged engine, the air is commonly compressedin a device called a blower. The blower is driven through gears directly from theengines crankshaft. The most common type of blower uses two rotating rotorsto compress the air. Supercharging is more commonly found on two-strokeengines where the higher pressures that a supercharger is capable of generatingare needed.

Exhaust System

The exhaust system of a diesel engine performs three functions. First, the exhaust systemroutes the spent combustion gasses away from the engine, where they are diluted by theatmosphere. This keeps the area around the engine habitable. Second, the exhaust systemconfines and routes the gasses to the turbocharger, if used. Third, the exhaust systemallows mufflers to be used to reduce the engine noise.

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Oper ational Ter minology

Before a detailed operation of a diesel engine can be explained, several terms must be defined.

Bor e and Stroke

Bore and stroke are terms used to define the size of an engine. As previously stated, borerefers to the diameter of the engine's cylinder, and stroke refers to the distance the pistontravels from the top of the cylinder to the bottom. The highest point of travel by thepiston is called top dead center (TDC), and the lowest point of travel is called bottomdead center (BDC). There are 180o of travel between TDC and BDC, or one stroke.

Engine Displacement

Engine displacement is one of the terms used to compare one engine to another.Displacement refers to the total volume displaced by all the pistons during one stroke.The displacement is usually given in cubic inches or liters. To calculate the displacementof an engine, the volume of one cylinder must be determined (volume of a cylinder =(πr2)h where h = the stroke). The volume of one cylinder is multiplied by the numberof cylinders to obtain the total engine displacement.

Degree of Cr ankshaft Rotation

All events that occur in an engine are related to the location of the piston. Because thepiston is connected to the crankshaft, any location of the piston corresponds directly toa specific number of degrees of crankshaft rotation.

Location of the crank can then be stated as XX degrees before or XX degrees after topor bottom dead center.

Fir ing Or der

Firing order refers to the order in which each of the cylinders in a multicylinder enginefires (power stroke). For example, a four cylinder engine's firing order could be 1-4-3-2.This means that the number 1 cylinder fires, then the number 4 cylinder fires, then thenumber 3 cylinder fires, and so on. Engines are designed so that the power strokes areas uniform as possible, that is, as the crankshaft rotates a certain number of degrees, oneof the cylinders will go through a power stroke. This reduces vibration and allows thepower generated by the engine to be applied to the load in a smoother fashion than if theywere all to fire at once or in odd multiples.

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Compression Ratio and Clearance Volume

Clearance volume is the volume remaining in the cylinder when the piston is at TDC.Because of the irregular shape of the combustion chamber (volume in the head) theclearance volume is calculated empirically by filling the chamber with a measured amountof fluid while the piston is at TDC. This volume is then added to the displacementvolume in the cylinder to obtain the cylinders total volume.

An engine's compression ratio is determined by taking the volume of the cylinder withpiston at TDC (highest point of travel) and dividing the volume of the cylinder when thepiston is at BDC (lowest point of travel), as shown in Figure 15. This can be calculatedby using the following formula:

Compression Ratio displacement volume clearance volumeclearance volume

Figure 15 Compression Ratio

Hor sepower

Power is the amount of work done per unit time or the rate of doing work. For a dieselengine, power is rated in units of horsepower. Indicated horsepower is the powertransmitted to the pistons by the gas in the cylinders and is mathematically calculated.

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Brake horsepower refers to the amount of usable power delivered by the engine to thecrankshaft. Indicated horsepower can be as much as 15% higher than brake horsepower.The difference is due to internal engine friction, combustion inefficiencies, and parasiticlosses, for example, oil pump, blower, water pump, etc.

The ratio of an engine's brake horsepower and its indicated horsepower is called themechanical efficiency of the engine. The mechanical efficiency of a four-cycle diesel isabout 82 to 90 percent. This is slightly lower than the efficiency of the two-cycle dieselengine. The lower mechanical efficiency is due to the additional friction losses and powerneeded to drive the piston through the extra 2 strokes.

Engines are rated not only in horsepower but also by the torque they produce. Torqueis a measure of the engine's ability to apply the power it is generating. Torque iscommonly given in units of lb-ft.

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Summary

The important information in this chapter is summarized below.

Diesel Engines Summary

The compression ratio is the volume of the cylinder with piston at

TDC divided by the volume of the cylinder with piston at BDC.

Bore is the diameter of the cylinder.

Stroke is the distance the piston travels from TDC to BDC, and is

determined by the eccentricity of the crankshaft.

The combustion chamber is the volume of space where the fuel air mixture

is burned in an engine. This is in the cylinder of the engine.

The following components were discussed and identified on a drawing.

a. Piston and rod

b. Cylinder

c. Blower

d. Crankshaft

e. Intake ports or valve(s)

f. Exhaust ports or valve(s)

g. Fuel injector

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DOE-HDBK-1018/1-93Diesel Engine Fundamentals FUNDAMENTALS OF THE DIESEL CYCLE

FUNDAMENTALS OF THE DIESEL CYCLE

Diesel engines operate under the principle of the internal combustion engine.There are two basic types of diesel engines, two-cycle and four-cycle. Anunderstanding of how each cycle operates is required to understand how tocorrectly operate and maintain a diesel engine.

EO 1.3 EXPLAIN how a diesel engine converts the chemical energystored in the diesel fuel into mechanical energy.

EO 1.4 EXPLAIN how the ignition process occurs in a diesel engine.

EO 1.5 EXPLAIN the operation of a 4-cycle diesel engine, includingwhen the following events occur during a cycle:


EO 1.6 EXPLAIN the operation of a 2-cycle diesel engine, includingwhen the following events occur during a cycle:


The Basic Diesel Cycles

A diesel engine is a type of heat engine that uses the internal combustion process to convert theenergy stored in the chemical bonds of the fuel into useful mechanical energy. This occurs intwo steps. First, the fuel reacts chemically (burns) and releases energy in the form of heat.Second the heat causes the gasses trapped in the cylinder to expand, and the expanding gases,being confined by the cylinder, must move the piston to expand. The reciprocating motion ofthe piston is then converted into rotational motion by the crankshaft.

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DOE-HDBK-1018/1-93FUNDAMENTALS OF THE DIESEL CYCLE Diesel Engine Fundamentals

To convert the chemical energy of the fuel into useful mechanical energy all internal combustionengines must go through four events: intake, compression, power, and exhaust. How theseevents are timed and how they occur differentiates the various types of engines.

All diesel engines fall into one of two categories, two-stroke or four-stroke cycle engines. Theword cycle refers to any operation or series of events that repeats itself. In the case of a four-stroke cycle engine, the engine requires four strokes of the piston (intake, compression, power,and exhaust) to complete one full cycle. Therefore, it requires two rotations of the crankshaft,or 720° of crankshaft rotation (360° x 2) to complete one cycle. In a two-stroke cycle enginethe events (intake, compression, power, and exhaust) occur in only one rotation of the crankshaft,or 360°.

T iming

In the following discussion of the diesel cycle it is important to keep in mind the timeframe in which each of the actions is required to occur. Time is required to move exhaustgas out of the cylinder and fresh air in to the cylinders, to compress the air, to inject fuel,and to burn the fuel. If a four-stroke diesel engine is running at a constant 2100revolutions per minute (rpm), the crankshaft would be rotating at 35 revolutions, or12,600 degrees, per second. One stroke is completed in about 0.01429 seconds.

The Four -Stoke Cycle

In a four-stroke engine the camshaft is geared so that it rotates at half the speed of the crankshaft

Figure 16 Scavenging and Intake

(1:2). This means that the crankshaft must make two complete revolutions before the camshaftwill complete one revolution. The following section will describe a four-stroke, normallyaspirated, diesel engine having both intake and exhaust valveswith a 3.5-inch bore and 4-inch stroke with a 16:1 compressionratio, as it passes through one complete cycle. We will start onthe intake stroke. All the timing marks given are generic andwill vary from engine to engine. Refer to Figures 10, 16, and 17during the following discussion.

I ntake

As the piston moves upward and approaches 28° beforetop dead center (BTDC), as measured by crankshaftrotation, the camshaft lobe starts to lift the cam follower.This causes the pushrod to move upward and pivots therocker arm on the rocker arm shaft. As the valve lash istaken up, the rocker arm pushes the intake valvedownward and the valve starts to open. The intakestroke now starts while the exhaust valve is still open.The flow of the exhaust gasses will have created a low

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pressure condition within the cylinder and will help pull in the fresh air charge as shownin Figure 16.

The piston continues its upward travel through top dead center (TDC) while fresh airenters and exhaust gasses leave. At about 12° after top dead center (ATDC), thecamshaft exhaust lobe rotates so that the exhaust valve will start to close. The valve isfully closed at 23° ATDC. This is accomplished through the valve spring, which wascompressed when the valve was opened, forcing the rocker arm and cam follower backagainst the cam lobe as it rotates. The time frame during which both the intake andexhaust valves are open is called valve overlap (51° of overlap in this example) and isnecessary to allow the fresh air to help scavenge (remove) the spent exhaust gasses andcool the cylinder. In most engines, 30 to 50 times cylinder volume is scavenged throughthe cylinder during overlap. This excess cool air also provides the necessary coolingeffect on the engine parts.

As the piston passes TDC and begins to travel down the cylinder bore, the movement ofthe piston creates a suction and continues to draw fresh air into the cylinder.

Compression

At 35° after bottom dead center (ABDC), the intake

Figure 17 Compression

valve starts to close. At 43° ABDC (or 137° BTDC),the intake valve is on its seat and is fully closed. Atthis point the air charge is at normal pressure (14.7 psia)and ambient air temperature (~80°F), as illustrated inFigure 17.

At about 70° BTDC, the piston has traveled about 2.125inches, or about half of its stroke, thus reducing thevolume in the cylinder by half. The temperature has nowdoubled to ~160°F and pressure is ~34 psia.

At about 43° BTDC the piston has traveled upward 3.062inches of its stroke and the volume is once again halved.Consequently, the temperature again doubles to about320°F and pressure is ~85 psia. When the piston hastraveled to 3.530 inches of its stroke the volume is againhalved and temperature reaches ~640°F and pressure 277 psia. When the piston hastraveled to 3.757 inches of its stroke, or the volume is again halved, the temperatureclimbs to 1280°F and pressure reaches 742 psia. With a piston area of 9.616 in2 thepressure in the cylinder is exerting a force of approximately 7135 lb. or 3-1/2 tons offorce.

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The above numbers are ideal and provide a good example of what is occurring in anengine during compression. In an actual engine, pressures reach only about 690 psia.This is due primarily to the heat loss to the surrounding engine parts.

Fuel In j ect ion

Figure 18 Fuel Injection

Fuel in a liquid state is injected into the cylinder ata precise time and rate to ensure that thecombustion pressure is forced on the piston neithertoo early nor too late, as shown in Figure 18. Thefuel enters the cylinder where the heatedcompressed air is present; however, it will onlyburn when it is in a vaporized state (attainedthrough the addition of heat to cause vaporization)and intimately mixed with a supply of oxygen.The first minute droplets of fuel enter thecombustion chamber and are quickly vaporized.The vaporization of the fuel causes the airsurrounding the fuel to cool and it requires timefor the air to reheat sufficiently to ignite thevaporized fuel. But once ignition has started, theadditional heat from combustion helps to furthervaporize the new fuel entering the chamber, as long as oxygen is present. Fuelinjection starts at 28° BTDC and ends at 3° ATDC; therefore, fuel is injected fora duration of 31°.

Power

Both valves are closed, and the fresh air charge has

Figure 19 Power

been compressed. The fuel has been injected andis starting to burn. After the piston passes TDC,heat is rapidly released by the ignition of the fuel,causing a rise in cylinder pressure. Combustiontemperatures are around 2336°F. This rise inpressure forces the piston downward and increasesthe force on the crankshaft for the power stroke asillustrated in Figure 19.

The energy generated by the combustion process isnot all harnessed. In a two stroke diesel engine,only about 38% of the generated power isharnessed to do work, about 30% is wasted in theform of heat rejected to the cooling system, andabout 32% in the form of heat is rejected out theexhaust. In comparison, the four-stroke dieselengine has a thermal distribution of 42% converted

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to useful work, 28% heat rejected to the cooling system, and 30% heat rejectedout the exhaust.

Exhaust

Figure 20 Exhaust

As the piston approaches 48° BBDC, the cam of theexhaust lobe starts to force the follower upward, causingthe exhaust valve to lift off its seat. As shown inFigure 20, the exhaust gasses start to flow out the exhaustvalve due to cylinder pressure and into the exhaustmanifold. After passing BDC, the piston moves upwardand accelerates to its maximum speed at 63° BTDC. Fromthis point on the piston is decelerating. As the pistonspeed slows down, the velocity of the gasses flowing outof the cylinder creates a pressure slightly lower thanatmospheric pressure. At 28° BTDC, the intake valveopens and the cycle starts again.

The Two-Stroke Cycle Like the four-stroke engine, the two-stroke engine must gothrough the same four events: intake, compression, power, and exhaust. But a two-stroke enginerequires only two strokes of the piston to complete one full cycle. Therefore, it requires only onerotation of the crankshaft to complete a cycle. This means several events must occur during eachstroke for all four events to be completed in two strokes, as opposed to the four-stroke enginewhere each stroke basically contains one event.

In a two-stroke engine the camshaft is geared so that it rotates at the same speed as thecrankshaft (1:1). The following section will describe a two-stroke, supercharged, diesel enginehaving intake ports and exhaust valves with a 3.5-inch bore and 4-inch stroke with a 16:1compression ratio, as it passes through one complete cycle. We will start on the exhaust stroke.All the timing marks given are generic and will vary from engine to engine.

Exhaust and I ntake

At 82° ATDC, with the piston near the end of its power stroke, the exhaust cam beginsto lift the exhaust valves follower. The valve lash is taken up, and 9° later (91° ATDC),the rocker arm forces the exhaust valve off its seat. The exhaust gasses start to escapeinto the exhaust manifold, as shown in Figure 21. Cylinder pressure starts to decrease.

After the piston travels three-quarters of its (down) stroke, or 132° ATDC of crankshaftrotation, the piston starts to uncover the inlet ports. As the exhaust valve is still open, theuncovering of the inlet ports lets the compressed fresh air enter the cylinder and helpscool the cylinder and scavenge the cylinder of the remaining exhaust gasses (Figure 22).Commonly, intake and exhaust occur over approximately 96° of crankshaft rotation.

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At 43° ABDC, the camshaft starts to close the exhaust valve. At 53° ABDC (117°BTDC), the camshaft has rotated sufficiently to allow the spring pressure to close theexhaust valve. Also, as the piston travels past 48°ABDC (5° after the exhaust valve startsclosing), the intake ports are closed off by the piston.

Figure 21 2-Stroke Exhaust Figure 22 2-Stroke Intake

Compression

After the exhaust valve is on its seat (53° ATDC), the temperature and pressure begin torise in nearly the same fashion as in the four-stroke engine. Figure 23 illustrates thecompression in a 2-stroke engine. At 23° BTDC the injector cam begins to lift theinjector follower and pushrod. Fuel injection continues until 6° BTDC (17 total degreesof injection), as illustrated in Figure 24.

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Figure 23 2-Stroke Compression Figure 24 2-Stroke Fuel Injection

Power

Figure 25 2-Stroke Power

The power stroke starts after the piston passes TDC.Figure 25 illustrates the power stroke which continuesuntil the piston reaches 91° ATDC, at which point theexhaust valves start to open and a new cycle begins.

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Summary


Fundamentals of the Diesel Cycle Summary

Ignition occurs in a diesel by injecting fuel into the air charge which has beenheated by compression to a temperature greater than the ignition point of thefuel.

A diesel engine converts the energy stored in the fuel's chemical bonds intomechanical energy by burning the fuel. The chemical reaction of burning thefuel liberates heat, which causes the gasses to expand, forcing the piston torotate the crankshaft.

A four-stroke engine requires two rotations of the crankshaft to complete onecycle. The event occur as follows:

Intake - the piston passes TDC, the intake valve(s) open and the fresh air isadmitted into the cylinder, the exhaust valve is still open for a few degreesto allow scavenging to occur.

Compression - after the piston passes BDC the intake valve closes and thepiston travels up to TDC (completion of the first crankshaft rotation).

Fuel injection - As the piston nears TDC on the compression stroke, thefuel is injected by the injectors and the fuel starts to burn, further heatingthe gasses in the cylinder.

Power - the piston passes TDC and the expanding gasses force the pistondown, rotating the crankshaft.

Exhaust - as the piston passes BDC the exhaust valves open and theexhaust gasses start to flow out of the cylinder. This continues as the pistontravels up to TDC, pumping the spent gasses out of the cylinder. At TDCthe second crankshaft rotation is complete.

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Fundamentals of the Diesel Cycle Summary (Cont.)

A two-stroke engine requires one rotation of the crankshaft to complete onecycle. The events occur as follows:

Intake - the piston is near BDC and exhaust is in progress. The intakevalve or ports open and the fresh air is forced in. The exhaust valves orports are closed and intake continues.

Compression - after both the exhaust and intake valves or ports are closed,the piston travels up towards TDC. The fresh air is heated by thecompression.

Fuel injection - near TDC the fuel is injected by the injectors and the fuelstarts to burn, further heating the gasses in the cylinder.

Power - the piston passes TDC and the expanding gasses force the pistondown, rotating the crankshaft.

Exhaust - as the piston approaches BDC the exhaust valves or ports openand the exhaust gasses start to flow out of the cylinder.

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DIESEL ENGINE SPEED, DOE-HDBK-1018/1-93 Diesel Engine FundamentalsFUEL CONTROLS, AND PROTECTION

DIESEL ENGINE SPEED, FUEL CONTROLS,AND PROTECTION

Understanding how diesel engines are controlled and the types of protectiveinstrumentation available is important for a complete understanding of theoperation of a diesel engine.

EO 1.7 DESCRIBE how the mechanical-hydraulic governor on adiesel engine controls engine speed.

EO 1.8 LIST five protective alarms usually found on mid-sized andlarger diesel engines.

Engine Contr ol

The control of a diesel engine is accomplished through several components: the camshaft, the fuelinjector, and the governor. The camshaft provides the timing needed to properly inject the fuel,the fuel injector provides the component that meters and injects the fuel, and the governorregulates the amount of fuel that the injector is to inject. Together, these three major componentsensure that the engine runs at the desired speed.

Fuel I njector s

Each cylinder has a fuel injector designed to meter and inject fuel into the cylinder at the properinstant. To accomplish this function, the injectors are actuated by the engine's camshaft. Thecamshaft provides the timing and pumping action used by the injector to inject the fuel. Theinjectors meter the amount of fuel injected into the cylinder on each stroke. The amount of fuelto be injected by each injector is set by a mechanical linkage called the fuel rack. The fuel rackposition is controlled by the engine's governor. The governor determines the amount of fuelrequired to maintain the desired engine speed and adjusts the amount to be injected by adjustingthe position of the fuel rack.

Each injector operates in the following manner. As illustrated in Figure 26, fuel under pressureenters the injector through the injector's filter cap and filter element. From the filter element thefuel travels down into the supply chamber (that area between the plunger bushing and the spilldeflector). The plunger operates up and down in the bushing, the bore of which is open to thefuel supply in the supply chamber by two funnel-shaped ports in the plunger bushing.

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Diesel Engine Fundamentals DOE-HDBK-1018/1-93 DIESEL ENGINE SPEED,FUEL CONTROLS, AND PROTECTION

Figure 26 Fuel Injector Cutaway

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The motion of the injector rocker arm (not shown) is transmitted to the plunger by the injectorfollower which bears against the follower spring. As the plunger moves downward underpressure of the injector rocker arm, a portion of the fuel trapped under the plunger is displacedinto the supply chamber through the lower port until the port is closed off by the lower end ofthe plunger. The fuel trapped below the plunger is then forced up through the central bore of theplunger and back out the upper port until the upper port is closed off by the downward motionof the plunger. With the upper and lower ports both closed off, the remaining fuel under theplunger is subjected to an increase in pressure by the downward motion of the plunger.

When sufficient pressure has built up, the injector valve is lifted off its seat and the fuel is forcedthrough small orifices in the spray tip and atomized into the combustion chamber. A checkvalve, mounted in the spray tip, prevents air in the combustion chamber from flowing back intothe fuel injector. The plunger is then returned back to its original position by the injectorfollower spring.

On the return upward movement of the plunger, the high pressure cylinder within the bushing isagain filled with fresh fuel oil through the ports. The constant circulation of fresh, cool fuelthrough the injector renews the fuel supply in the chamber and helps cool the injector. The fuelflow also effectively removes all traces of air that might otherwise accumulate in the system.

The fuel injector outlet opening, through which the excess fuel returns to the fuel return manifoldand then back to the fuel tank, is adjacent to the inlet opening and contains a filter elementexactly the same as the one on the fuel inlet side.

In addition to the reciprocating motion of the plunger, the plunger can be rotated during operationaround its axis by the gear which meshes with the fuel rack. For metering the fuel, an upperhelix and a lower helix are machined in the lower part of the plunger. The relation of the helicesto the two ports in the injector bushing changes with the rotation of the plunger.

Changing the position of the helices, by rotating the plunger, retards or advances the closing ofthe ports and the beginning and ending of the injection period. At the same time, it increases ordecreases the amount of fuel injected into the cylinder. Figure 27 illustrates the various plungerpositions from NO LOAD to FULL LOAD. With the control rack pulled all the way (noinjection), the upper port is not closed by the helix until after the lower port is uncovered.Consequently, with the rack in this position, all of the fuel is forced back into the supplychamber and no injection of fuel takes place. With the control rack pushed all the way in (fullinjection), the upper port is closed shortly after the lower port has been covered, thus producinga maximum effective stroke and maximum fuel injection. From this no-injection position to thefull-injection position (full rack movement), the contour of the upper helix advances the closingof the ports and the beginning of injection.

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Figure 27 Fuel Injector Plunger

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Gover nor

Diesel engine speed is controlled solely by the amount of fuel injected into the engine by theinjectors. Because a diesel engine is not self-speed-limiting, it requires not only a means ofchanging engine speed (throttle control) but also a means of maintaining the desired speed. Thegovernor provides the engine with the feedback mechanism to change speed as needed and tomaintain a speed once reached.

A governor is essentially a speed-sensitive device, designed to maintain a constant engine speedregardless of load variation. Since all governors used on diesel engines control engine speedthrough the regulation of the quantity of fuel delivered to the cylinders, these governors may beclassified as speed-regulating governors. As with the engines themselves there are many typesand variations of governors. In this module, only the common mechanical-hydraulic typegovernor will be reviewed.

The major function of the governor is determined by the application of the engine. In an enginethat is required to come up and run at only a single speed regardless of load, the governor iscalled a constant-speed type governor. If the engine is manually controlled, or controlled by anoutside device with engine speed being controlled over a range, the governor is called a variable-speed type governor. If the engine governor is designed to keep the engine speed above aminimum and below a maximum, then the governor is a speed-limiting type. The last categoryof governor is the load limiting type. This type of governor limits fuel to ensure that the engineis not loaded above a specified limit. Note that many governors act to perform several of thesefunctions simultaneously.

Oper ation of a Gover nor

The following is an explanation of the operation of a constant speed, hydraulically compensatedgovernor using the Woodward brand governor as an example. The principles involved arecommon in any mechanical and hydraulic governor.

The Woodward speed governor operates the diesel engine fuel racks to ensure a constant enginespeed is maintained at any load. The governor is a mechanical-hydraulic type governor andreceives its supply of oil from the engine lubricating system. This means that a loss of lube oilpressure will cut off the supply of oil to the governor and cause the governor to shut down theengine. This provides the engine with a built-in shutdown device to protect the engine in theevent of loss of lubricating oil pressure.

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Simplif ied Oper ation of the Gover nor

The governor controls the fuel rack position through a combined action of the hydraulicpiston and a set of mechanical flyweights, which are driven by the engine blower shaft.Figure 28 provides an illustration of a functional diagram of a mechanical-hydraulicgovernor. The position of the flyweights is determined by the speed of the engine. Asthe engine speeds up or down, the weights move in or out. The movement of theflyweights, due to a change in engine speed, moves a small piston (pilot valve) in thegovernor's hydraulic system. This motion adjusts flow of hydraulic fluid to a largehydraulic piston (servo-motor piston). The large hydraulic piston is linked to the fuelrack and its motion resets the fuel rack for increased/decreased fuel.

Figure 28 Simplified Mechanical-Hydraulic Governor

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Detailed Oper ation of the Gover nor

With the engine operating, oil from the engine lubrication system is supplied to thegovernor pump gears, as illustrated in Figure 29. The pump gears raise the oil pressureto a value determined by the spring relief valve. The oil pressure is maintained in theannular space between the undercut portion of the pilot valve plunger and the bore in thepilot valve bushing. For any given speed setting, the spring speeder exerts a force thatis opposed by the centrifugal force of the revolving flyweights. When the two forces areequal, the control land on the pilot valve plunger covers the lower ports in the pilot valvebushing.

Figure 29 Cutaway of a Woodward Governor

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Under these conditions, equal oil pressures are maintained on both sides of the bufferpiston and tension on the two buffer springs is equal. Also, the oil pressure is equal onboth sides of the receiving compensating land of the pilot valve plunger due to oil passingthrough the compensating needle valve. Thus, the hydraulic system is in balance, and theengine speed remains constant.

When the engine load increases, the engine starts to slow down in speed. The reductionin engine speed will be sensed by the governor flyweights. The flyweights are forcedinward (by the spring), thus lowering the pilot valve plunger (again, due to the downwardspring force). Oil under pressure will be admitted under the servo-motor piston (topsideof the buffer piston) causing it to rise. This upward motion of the servo-motor piston willbe transmitted through the terminal lever to the fuel racks, thus increasing the amount offuel injected into the engine. The oil that forces the servo-motor piston upward alsoforces the buffer piston upward because the oil pressure on each side of the piston isunequal. This upward motion of the piston compresses the upper buffer spring andrelieves the pressure on the lower buffer spring.

The oil cavities above and below the buffer piston are common to the receivingcompensating land on the pilot valve plunger. Because the higher pressure is below thecompensating land, the pilot valve plunger is forced upward, recentering the flyweightsand causing the control land of the pilot valve to close off the regulating port. Thus, theupward movement of the servo-motor piston stops when it has moved far enough to makethe necessary fuel correction.

Oil passing through the compensating needle valve slowly equalizes the pressures aboveand below the buffer piston, thus allowing the buffer piston to return to the centerposition, which in turn equalizes the pressure above and below the receivingcompensating land. The pilot valve plunger then moves to its central position and theengine speed returns to its original setting because there is no longer any excessiveoutward force on the flyweights.

The action of the flyweights and the hydraulic feedback mechanism produces stableengine operation by permitting the governor to move instantaneously in response to theload change and to make the necessary fuel adjustment to maintain the initial enginespeed.

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Starting Cir cuits

Diesel engines have as many different types of starting circuits as there are types, sizes, andmanufacturers of diesel engines. Commonly, they can be started by air motors, electric motors,hydraulic motors, and manually. The start circuit can be a simple manual start pushbutton, ora complex auto-start circuit. But in almost all cases the following events must occur for thestarting engine to start.

1. The start signal is sent to the starting motor. The air, electric, or hydraulic motor,will engage the engine's flywheel.

2. The starting motor will crank the engine. The starting motor will spin the engineat a high enough rpm to allow the engine's compression to ignite the fuel and startthe engine running.

3. The engine will then accelerate to idle speed. When the starter motor is overdrivenby the running motor it will disengage the flywheel.

Because a diesel engine relies on compression heat to ignite the fuel, a cold engine can robenough heat from the gasses that the compressed air falls below the ignition temperature of thefuel. To help overcome this condition, some engines (usually small to medium sized engines)have glowplugs. Glowplugs are located in the cylinder head of the combustion chamber and useelectricity to heat up the electrode at the top of the glowplug. The heat added by the glowplugis sufficient to help ignite the fuel in the cold engine. Once the engine is running, the glowplugsare turned off and the heat of combustion is sufficient to heat the block and keep the enginerunning.

Larger engines usually heat the block and/or have powerful starting motors that are able to spinthe engine long enough to allow the compression heat to fire the engine. Some large engines useair start manifolds that inject compressed air into the cylinders which rotates the engine duringthe start sequence.

Engine Pr otection

A diesel engine is designed with protection systems to alert the operators of abnormal conditionsand to prevent the engine from destroying itself.

Overspeed device - Because a diesel is not self-speed-limiting, a failure in the governor,injection system, or sudden loss of load could cause the diesel tooverspeed. An overspeed condition is extremely dangerous becauseengine failure is usually catastrophic and can possibly cause the engine tofly apart.

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An overspeed device, usually some type of mechanical flyweight, will actto cut off fuel to the engine and alarm at a certain preset rpm. This isusually accomplished by isolating the governor from its oil supply, causingit to travel to the no-fuel position, or it can override the governor anddirectly trip the fuel rack to the no-fuel position.

Water jacket - Water-cooled engines can overheat if the cooling water system fails toremove waste heat. Removal of the waste heat prevents the engine fromseizing due to excessive expansion of the components under a hightemperature condition. The cooling water jacket is commonly where thesensor for the cooling water system is located.

The water jacket temperature sensors provide early warning of abnormalengine temperature, usually an alarm function only. The setpoint is setsuch that if the condition is corrected in a timely manner, significantengine damage will be avoided. But continued engine operation at thealarm temperature or higher temperatures will lead to engine damage.

Exhaust In a diesel engine, exhaust temperatures are very important and cantemperatures - provide a vast amount of information regarding the operation of the

engine. High exhaust temperature can indicate an overloading of theengine or possible poor performance due to inadequate scavenging (thecooling effect) in the engine. Extended operation with high exhausttemperatures can result in damage to the exhaust valves, piston, andcylinders. The exhaust temperature usually provides only an alarmfunction.

Low lube oil Low oil pressure or loss of oil pressure can destroy an engine in shortpressure - order. Therefore, most medium to larger engines will stop upon low or

loss of oil pressure. Loss of oil pressure can result in the engine seizingdue to lack of lubrication. Engines with mechanical-hydraulic governorswill also stop due to the lack of oil to the governor.

The oil pressure sensor usually stops the engine. The oil pressure sensorson larger engines usually have two low pressure setpoints. One setpointprovides early warning of abnormal oil pressure, an alarm function only.The second setpoint can be set to shutdown the engine before permanentdamage is done.

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High crankcase High crankcase pressure is usually caused by excessive blow-by (gaspressure - pressure in the cylinder blowing by the piston rings and into the

crankcase). The high pressure condition indicates the engine is in poorcondition. The high crankcase pressure is usually used only as an alarmfunction.

Summary


Diesel Engine Speed, Fuel Controls, and Protection Summary

A mechanical-hydraulic governor controls engine speed by balancing

engine speed (mechanical flyweights) against hydraulic pressure. As the

engine speeds up or slows down, the weights move the hydraulic plunger

in or out. This in turn actuates a hydraulic valve which controls the

hydraulic pressure to the buffer piston. The buffer piston is connected to

the fuel rack. Therefore, any motion of the buffer piston will control fuel

to the cylinder by adjusting the position of the fuel rack, which regulates

the amount of fuel in the injectors.

Most mid-sized to large diesel engines have (as a minimum) the following

protective alarms and trips.

Engine overspeed alarm/trip

High water jacket temperature alarm

High exhaust temperature alarm

Low lube oil pressure (alarm and/or trip)

High crankcase pressure alarm

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Heat Exchangers


Heat Exchangers DOE-HDBK-1018/1-93 TABLE OF CONTENTS

TABLE OF CONTENTS


LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

TYPES OF HEAT EXCHANGERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Types of Heat Exchanger Construction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Types of Heat Exchangers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Comparison of the Types of Heat Exchangers. . . . . . . . . . . . . . . . . . . . . . . . . . 6Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

HEAT EXCHANGER APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Preheater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Radiator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Air Conditioner Evaporator and Condenser. . . . . . . . . . . . . . . . . . . . . . . . . . . 14Large Steam System Condensers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Rev. 0 Page i ME-02


LIST OF FIGURES DOE-HDBK-1018/1-93 Heat Exchangers

LIST OF FIG URES

Figure 1 Tube and Shell Heat Exchanger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 2 Plate Heat Exchanger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 3 Parallel Flow Heat Exchanger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 4 Counter Flow Heat Exchange. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 5 Cross Flow Heat Exchanger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 6 Single and Multi-Pass Heat Exchangers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 7 Regenerative and Non-Regenerative Heat Exchangers. . . . . . . . . . . . . . . . . . 10

Figure 8 U-tube Feedwater Heat Exchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 9 Single Pass Condenser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 10 Jet Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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Heat Exchangers DOE-HDBK-1018/1-93 LIST OF TABLES

LIST OF TABLES

NONE

Rev. 0 Page iii ME-02


REFERENCES DOE-HDBK-1018/1-93 Heat Exchangers

REFERENCES

Babcock & Wilcox, Steam, Its Generations and Use, Babcock & Wilcox Co.


Heat Transfer, Thermodynamics and Fluid Flow Fundamentals, Columbia, MD,General Physics Corporation, Library of Congress Card #A 326517.

Marley, Cooling Tower Fundamentals and Applications, The Marley Company.

ME-02 Page iv Rev. 0


Heat Exchangers DOE-HDBK-1018/1-93 OBJECTIVES

TERMINAL OBJECTIVE

1.0 Without references, DESCRIBE the purpose, construction, and principles of operation foreach major type of heat exchanger: parallel flow, counter flow, and cross flow.

ENABLING OBJECTIVES

1.1 STATE the two types of heat exchanger construction.

1.2 Provided with a drawing of a heat exchanger, IDENTIFY the following internal parts:

a. Tubesb. Tube sheetc. Shelld. Baffles

1.3 DESCRIBE hot and cold fluid flow in parallel flow, counter flow, and cross flow heatexchangers.

1.4 DIFFERENTIATE between the following types of heat exchangers:

a. Single-pass versus multi-pass heat exchangers.b. Regenerative versus non-regenerative heat exchangers.

1.5 LIST at least three applications of heat exchangers.

1.6 STATE the purpose of a condenser.

1.7 DEFINE the following terms:

a. Hotwellb. Condensate depression

1.8 STATE why condensers in large steam cycles are operated at a vacuum.

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OBJECTIVES DOE-HDBK-1018/1-93 Heat Exchangers

Intentionally Left Blank

ME-02 Page vi Rev. 0


Heat Exchangers DOE-HDBK-1018/1-93 TYPES OF HEAT EXCHANGERS

TYPES OF HEAT EXCHANGERS

In almost any nuclear, chemical, or mechanical system, heat must be transferredfrom one place to another or from one fluid to another. Heat exchangers are usedto transfer heat from one fluid to another. A basic understanding of themechanical components of a heat exchanger is important to understanding howthey function and operate.

EO 1.1 STATE the two types of heat exchanger construction.

EO 1.2 Provided with a drawing of a heat exchanger, IDENTIFY thefollowing internal parts:

a. Tubes c. Shellb. Tube sheet d. Baffles

EO 1.3 DESCRIBE hot and cold fluid flow in parallel flow, counterflow, and cross flow heat exchangers.

EO 1.4 DIFFERENTIATE between the following types of heat exchangers:

a. Single-pass versus multi-pass heat exchangersb. Regenerative versus non-regenerative heat exchangers

I ntr oduction

A heat exchanger is a component that allows the transfer of heat from one fluid (liquid or gas)to another fluid. Reasons for heat transfer include the following:

1. To heat a cooler fluid by means of a hotter fluid

2. To reduce the temperature of a hot fluid by means of a cooler fluid

3. To boil a liquid by means of a hotter fluid

4. To condense a gaseous fluid by means of a cooler fluid

5. To boil a liquid while condensing a hotter gaseous fluid

Regardless of the function the heat exchanger fulfills, in order to transfer heat the fluids involvedmust be at different temperatures and they must come into thermal contact. Heat can flow onlyfrom the hotter to the cooler fluid.

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TYPES OF HEAT EXCHANGERS DOE-HDBK-1018/1-93 Heat Exchangers

In a heat exchanger there is no direct contact between the two fluids. The heat is transferredfrom the hot fluid to the metal isolating the two fluids and then to the cooler fluid.

Types of Heat Exchanger Constr uction

Although heat exchangers come in every shape and size imaginable, the construction of most heatexchangers fall into one of two categories: tube and shell, or plate. As in all mechanical devices,each type has its advantages and disadvantages.

Tube and Shell

The most basic and the most common type of heat exchanger construction is the tube andshell, as shown in Figure 1. This type of heat exchanger consists of a set of tubes in acontainer called a shell. The fluid flowing inside the tubes is called the tube side fluidand the fluid flowing on the outside of the tubes is the shell side fluid. At the ends ofthe tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s).The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tightseal. In systems where the two fluids are at vastly different pressures, the higher pressurefluid is typically directed through the tubes and the lower pressure fluid is circulated onthe shell side. This is due to economy, because the heat exchanger tubes can be madeto withstand higher pressures than the shell of the heat exchanger for a much lower cost.The support plates shown on Figure 1 also act as baffles to direct the flow of fluid withinthe shell back and forth across the tubes.

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Plate

Figure 1 Tube and Shell Heat Exchanger

A plate type heat exchanger, as illustrated in Figure 2, consists of plates instead of tubesto separate the hot and cold fluids. The hot and cold fluids alternate between each of theplates. Baffles direct the flow of fluid between plates. Because each of the plates hasa very large surface area, the plates provide each of the fluids with an extremely largeheat transfer area. Therefore a plate type heat exchanger, as compared to a similarlysized tube and shell heat exchanger, is capable of transferring much more heat. This isdue to the larger area the plates provide over tubes. Due to the high heat transferefficiency of the plates, plate type heat exchangers are usually very small when comparedto a tube and shell type heat exchanger with the same heat transfer capacity. Plate typeheat exchangers are not widely used because of the inability to reliably seal the largegaskets between each of the plates. Because of this problem, plate type heat exchangershave only been used in small, low pressure applications such as on oil coolers forengines. However, new improvements in gasket design and overall heat exchangerdesign have allowed some large scale applications of the plate type heat exchanger. Asolder facilities are upgraded or newly designed facilities are built, large plate type heatexchangers are replacing tube and shell heat exchangers and becoming more common.

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Types of Heat Exchanger s

Figure 2 Plate Heat Exchanger

Because heat exchangers come in so many shapes, sizes, makes, and models, they are categorizedaccording to common characteristics. One common characteristic that can be used to categorizethem is the direction of flow the two fluids have relative to each other. The three categories areparallel flow, counter flow and cross flow.

Parallel flow, as illustrated in Figure 3, exists when both the tube side fluid and the shellside fluid flow in the same direction. In this case, the two fluids enter the heatexchanger from the same end with a large temperature difference. As the fluids transferheat, hotter to cooler, the temperatures of the two fluids approach each other. Note thatthe hottest cold-fluid temperature is always less than the coldest hot-fluid temperature.

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Figure 3 Parallel Flow Heat Exchanger

Counter flow, as illustrated in Figure 4, exists when the two fluids flow in oppositedirections. Each of the fluids enters the heat exchanger at opposite ends. Because thecooler fluid exits the counter flow heat exchanger at the end where the hot fluid entersthe heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid.Counter flow heat exchangers are the most efficient of the three types. In contrast to theparallel flow heat exchanger, the counter flow heat exchanger can have the hottest cold-fluid temperature greater than the coldest hot-fluid temperatue.

Figure 4 Counter Flow Heat Exchange

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Cross flow, as illustrated in Figure 5, exists when one fluid flows perpendicular to thesecond fluid; that is, one fluid flows through tubes and the second fluid passes around thetubes at 90° angle. Cross flow heat exchangers are usually found in applications whereone of the fluids changes state (2-phase flow). An example is a steam system'scondenser, in which the steam exiting the turbine enters the condenser shell side, and thecool water flowing in the tubes absorbs the heat from the steam, condensing it into water.Large volumes of vapor may be condensed using this type of heat exchanger flow.

Figure 5 Cross Flow Heat Exchanger

Comparison of the Types of Heat Exchanger s

Each of the three types of heat exchangers has advantages and disadvantages. But of the three,the counter flow heat exchanger design is the most efficient when comparing heat transfer rateper unit surface area. The efficiency of a counter flow heat exchanger is due to the fact that theaverage T (difference in temperature) between the two fluids over the length of the heatexchanger is maximized, as shown in Figure 4. Therefore the log mean temperature for acounter flow heat exchanger is larger than the log mean temperature for a similar parallel orcross flow heat exchanger. (See the Thermodynamics, Heat Transfer, and Fluid FlowFundamentals Handbook for a review of log mean temperature). This can be seen by comparingthe graphs in Figure 3, Figure 4, and Figure 5. The following exercise demonstrates how thehigher log mean temperature of the counter flow heat exchanger results in a larger heat transferrate. The log mean temperature for a heat exchanger is calculated using the following equation.

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(2-1)∆ Tlm ∆ T2 ∆ T1

ln ∆ T2

∆ T1

Heat transfer in a heat exchanger is by conduction and convection. The rate of heattransfer, "Q", in a heat exchanger is calculated using the following equation.

(2-2) Q UoAo∆ Tlm

Where:

= Heat transfer rate (BTU/hr) Q

Uo = Overall heat transfer coefficient (BTU/hr-ft2-°F)

Ao = Cross sectional heat transfer area (ft2)

∆Tlm = Log mean temperature difference (°F)

Consider the following example of a heat exchanger operated under identical conditions as acounter flow and then a parallel flow heat exchanger.

T1 = represents the hot fluid temperature

T1in = 200°F

T1out = 145°F

Uo = 70 BTU/hr-ft2-°F

Ao = 75ft2

T2 = represents the cold fluid temperature

T2in = 80°F

T2out = 120°F

Counter flow ∆Tlm = (200 120oF) (145 80oF)

ln (200 120oF)

(145 80oF)

72oF

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Parallel flow ∆Tlm = (200 80oF) (145 120oF)

ln (200 80oF)

(145 120oF)

61oF

Inserting the above values into heat transfer Equation (2-2) for the counter flow heatexchanger yields the following result.

Q

70 BTU

hr ft 2 F(75ft 2) (72 F)

Q 3.8x105 BTUhr

Inserting the above values into the heat transfer Equation (2-2) for parallel flow heatexchanger yields the following result.

Q

70 BTU

hr ft 2 F(75ft 2) (61 F)

Q 3.2x105 BTUhr

The results demonstrate that given the same operating conditions, operating the same heatexchanger in a counter flow manner will result in a greater heat transfer rate thanoperating in parallel flow.

In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow;they are usually a combination of the two or all three types of heat exchangers. This is due tothe fact that actual heat exchangers are more complex than the simple components shown in theidealized figures used above to depict each type of heat exchanger. The reason for thecombination of the various types is to maximize the efficiency of the heat exchanger within therestrictions placed on the design. That is, size, cost, weight, required efficiency, type of fluids,operating pressures, and temperatures, all help determine the complexity of a specific heatexchanger.

One method that combines the characteristics of two or more heat exchangers and improves theperformance of a heat exchanger is to have the two fluids pass each other several times withina single heat exchanger. When a heat exchanger's fluids pass each other more than once, a heatexchanger is called a multi-pass heat exchanger. If the fluids pass each other only once, the heatexchanger is called a single-pass heat exchanger. See Figure 6 for an example of both types.Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or moresets of "U" bends in the tubes. The "U" bends allow the fluid to flow back and forth across thelength of the heat exchanger. A second method to achieve multiple passes is to insert baffleson the shell side of the heat exchanger. These direct the shell side fluid back and forth acrossthe tubes to achieve the multi-pass effect.

ME-02 Rev. 0Page 8



Figure 6 Single and Multi-Pass Heat Exchangers

Heat exchangers are also classified by their function in a particular system. One commonclassification is regenerative or nonregenerative. A regenerative heat exchanger is one in whichthe same fluid is both the cooling fluid and the cooled fluid, as illustrated in Figure 7. That is,the hot fluid leaving a system gives up its heat to "regenerate" or heat up the fluid returning tothe system. Regenerative heat exchangers are usually found in high temperature systems wherea portion of the system's fluid is removed from the main process, and then returned. Becausethe fluid removed from the main process contains energy (heat), the heat from the fluid leavingthe main system is used to reheat (regenerate) the returning fluid instead of being rejected to anexternal cooling medium to improve efficiency. It is important to remember that the termregenerative/nonregenerative only refers to "how" a heat exchanger functions in a system, anddoes not indicate any single type (tube and shell, plate, parallel flow, counter flow, etc.).

Rev. 0 ME-02Page 9



In a nonregenerative heat exchanger, as illustrated in Figure 7, the hot fluid is cooled by fluidfrom a separate system and the energy (heat) removed is not returned to the system.

Figure 7 Regenerative and Non-Regenerative Heat Exchangers

ME-02 Rev. 0Page 10



Summary

The important information from this chapter is summarized below.

Types of Heat Exchangers Summary

There are two methods of constructing heat exchangers:plate type and tube type.

Parallel flow - the hot fluid and the coolant flow in thesame direction.

Counter flow - The hot fluid and the coolant flow inopposite directions.

Cross flow - the hot fluid and the coolant flow at 90°angles (perpendicular) to each other.

The four heat exchanger parts identified were:

TubesTube SheetShellBaffles

Single-pass heat exchangers have fluids that pass eachother only once.

Multi-pass heat exchangers have fluids that pass each othermore than once through the use of U tubes and baffles.

Regenerative heat exchangers use the same fluid forheating and cooling.

Non-regenerative heat exchangers use separate fluids forheating and cooling.

Rev. 0 ME-02Page 11


HEAT EXCHANGER APPLICATIONS DOE-HDBK-1018/1-93 Heat Exchangers

HEAT EXCHANGER APPLICATIONS

This chapter describes some specific applications of heat exchangers.

EO 1.5 LIST at least three applications of heat exchangers.

EO 1.6 STATE the purpose of a condenser.

EO 1.7 DEFINE the following terms:

a. Hotwellb. Condensate depression

EO 1.8 STATE why condensers in large steam cycles areoperated at a vacuum.

I ntr oduction

Heat exchangers are found in most chemical or mechanical systems. They serve as the system'smeans of gaining or rejecting heat. Some of the more common applications are found inheating, ventilation and air conditioning (HVAC) systems, radiators on internal combustionengines, boilers, condensers, and as preheaters or coolers in fluid systems. This chapter willreview some specific heat exchanger applications. The intent is to provide several specificexamples of how each heat exchanger functions in the system, not to cover every possibleapplicaton.

Pr eheater

In large steam systems, or in any process requiring high temperatures, the input fluid is usuallypreheated in stages, instead of trying to heat it in one step from ambient to the final temperature.Preheating in stages increases the plant's efficiency and minimizes thermal shock stress tocomponents, as compared to injecting ambient temperature liquid into a boiler or other devicethat operates at high temperatures. In the case of a steam system, a portion of the process steamis tapped off and used as a heat source to reheat the feedwater in preheater stages. Figure 8 isan example of the construction and internals of a U-tube feedwater heat exchanger found in alarge power generation facility in a preheater stage. As the steam enters the heat exchanger andflows over and around the tubes, it transfers its thermal energy and is condensed. Note that thesteam enters from the top into the shell side of the heat exchanger, where it not only transferssensible heat (temperature change) but also gives up its latent heat of vaporization (condensessteam into water). The condensed steam then exits as a liquid at the bottom of the heatexchanger. The feedwater enters the heat exchanger on the bottom right end and flows into thetubes. Note that most of these tubes will be below the fluid level on the shell side.

ME-02 Rev. 0Page 12


Heat Exchangers DOE-HDBK-1018/1-93 HEAT EXCHANGER APPLICATIONS

This means the feedwater is exposed to the condensed steam first and then travels through thetubes and back around to the top right end of the heat exchanger. After making the 180° bend,the partially heated feedwater is then subjected to the hotter steam entering the shell side.

Figure 8 U-tube Feedwater Heat Exchanger

The feedwater is further heated by the hot steam and then exits the heat exchanger. In this typeof heat exchanger, the shell side fluid level is very important in determining the efficiency ofthe heat exchanger, as the shell side fluid level determines the number of tubes exposed to thehot steam.

Radiator

Commonly, heat exchangers are thought of as liquid-to-liquid devices only. But a heatexchanger is any device that transfers heat from one fluid to another. Some of a facility'sequipment depend on air-to-liquid heat exchangers. The most familiar example of an air-to-liquid heat exchanger is a car radiator. The coolant flowing in the engine picks up heat fromthe engine block and carries it to the radiator. From the radiator, the hot coolant flows into thetube side of the radiator (heat exchanger). The relatively cool air flowing over the outside of thetubes picks up the heat, reducing the temperature of the coolant.

Rev. 0 ME-02Page 13



Because air is such a poor conductor of heat, the heat transfer area between the metal of theradiator and the air must be maximized. This is done by using fins on the outside of the tubes.The fins improve the efficiency of a heat exchanger and are commonly found on most liquid-to-air heat exchangers and in some high efficiency liquid-to-liquid heat exchangers.

Air Conditioner Evapor ator and Condenser

All air conditioning systems contain at least two heat exchangers, usually called the evaporatorand the condenser. In either case, evaporator or condenser, the refrigerant flows into the heatexchanger and transfers heat, either gaining or releasing it to the cooling medium. Commonly,the cooling medium is air or water. In the case of the condenser, the hot, high pressurerefrigerant gas must be condensed to a subcooled liquid.

The condenser accomplishes this by cooling the gas, transferring its heat to either air or water.The cooled gas then condenses into a liquid. In the evaporator, the subcooled refrigerant flowsinto the heat exchanger, but the heat flow is reversed, with the relatively cool refrigerantabsorbing heat from the hotter air flowing on the outside of the tubes. This cools the air andboils the refrigerant.

L arge Steam System Condenser s

The steam condenser, shown in Figure 9, is a major component of the steam cycle in powergeneration facilities. It is a closed space into which the steam exits the turbine and is forced togive up its latent heat of vaporization. It is a necessary component of the steam cycle for tworeasons. One, it converts the used steam back into water for return to the steam generator orboiler as feedwater. This lowers the operational cost of the plant by allowing the clean andtreated condensate to be reused, and it is far easier to pump a liquid than steam. Two, itincreases the cycle's efficiency by allowing the cycle to operate with the largest possible delta-T and delta-P between the source (boiler) and the heat sink (condenser).

Because condensation is taking place, the term latent heat of condensation is used instead oflatent heat of vaporization. The steam's latent heat of condensation is passed to the waterflowing through the tubes of the condenser.

After the steam condenses, the saturated liquid continues to transfer heat to the cooling wateras it falls to the bottom of the condenser, or hotwell. This is called subcooling, and a certainamount is desirable. A few degrees subcooling prevents condensate pump cavitation. Thedifference between the saturation temperature for the existing condenser vacuum and thetemperature of the condensate is termed condensate depression. This is expressed as a numberof degrees condensate depression or degrees subcooled. Excessive condensate depressiondecreases the operating efficiency of the plant because the subcooled condensate must bereheated in the boiler, which in turn requires more heat from the reactor, fossil fuel, or other heatsource.

ME-02 Rev. 0Page 14



Figure 9 Single-Pass Condenser

There are different condenser designs, but the most common, at least in the large powergeneration facilities, is the straight-through, single-pass condenser illustrated Figure 9. Thiscondenser design provides cooling water flow through straight tubes from the inlet water boxon one end, to the outlet water box on the other end. The cooling water flows once through thecondenser and is termed a single pass. The separation between the water box areas and thesteam condensing area is accomplished by a tube sheet to which the cooling water tubes areattached. The cooling water tubes are supported within the condenser by the tube support sheets.Condensers normally have a series of baffles that redirect the steam to minimize directimpingement on the cooling water tubes. The bottom area of the condenser is the hotwell, asshown in Figure 9. This is where the condensate collects and the condensate pump takes itssuction. If noncondensable gasses are allowed to build up in the condenser, vacuum willdecrease and the saturation temperature at which the steam will condense increases.

Non-condensable gasses also blanket the tubes of the condenser, thus reducing the heat transfersurface area of the condenser. This surface area can also be reduced if the condensate level isallowed to rise over the lower tubes of the condenser. A reduction in the heat transfer surfacehas the same effect as a reduction in cooling water flow. If the condenser is operating near itsdesign capacity, a reduction in the effective surface area results in difficulty maintainingcondenser vacuum.

The temperature and flow rate of the cooling water through the condenser controls thetemperature of the condensate. This in turn controls the saturation pressure (vacuum) of thecondenser.

Rev. 0 ME-02Page 15



To prevent the condensate level from rising to the lower tubes of the condenser, a hotwell levelcontrol system may be employed. Varying the flow of the condensate pumps is one method usedto accomplish hotwell level control. A level sensing network controls the condensate pumpspeed or pump discharge flow control valve position. Another method employs an overflowsystem that spills water from the hotwell when a high level is reached.

Condenser vacuum should be maintained as close to 29 inches Hg as practical. This allowsmaximum expansion of the steam, and therefore, the maximum work. If the condenser wereperfectly air-tight (no air or noncondensable gasses present in the exhaust steam), it would benecessary only to condense the steam and remove the condensate to create and maintain avacuum. The sudden reduction in steam volume, as it condenses, would maintain the vacuum.Pumping the water from the condenser as fast as it is formed would maintain the vacuum. Itis, however, impossible to prevent the entrance of air and other noncondensable gasses into thecondenser. In addition, some method must exist to initially cause a vacuum to exist in thecondenser. This necessitates the use of an air ejector or vacuum pump to establish and helpmaintain condenser vacuum.

Air ejectors are essentially jet pumps or eductors, as illustrated in Figure 10. In operation, thejet pump has two types of fluids. They are the high pressure fluid that flows through the nozzle,and the fluid being pumped which flows around the nozzle into the throat of the diffuser. Thehigh velocity fluid enters the diffuser where its molecules strike other molecules. Thesemolecules are in turn carried along with the high velocity fluid out of the diffuser creating a lowpressure area around the mouth of the nozzle. This process is called entrainment. The lowpressure area will draw more fluid from around the nozzle into the throat of the diffuser. As thefluid moves down the diffuser, the increasing area converts the velocity back to pressure. Useof steam at a pressure between 200 psi and 300 psi as the high pressure fluid enables a single-stage air ejector to draw a vacuum of about 26 inches Hg.

Figure 10 Jet Pump

ME-02 Rev. 0Page 16



Normally, air ejectors consist of two suction stages. The first stage suction is located on top ofthe condenser, while the second stage suction comes from the diffuser of the first stage. Theexhaust steam from the second stage must be condensed. This is normally accomplished by anair ejector condenser that is cooled by condensate. The air ejector condenser also preheats thecondensate returning to the boiler. Two-stage air ejectors are capable of drawing vacuums to29 inches Hg.

A vacuum pump may be any type of motor-driven air compressor. Its suction is attached to thecondenser, and it discharges to the atmosphere. A common type uses rotating vanes in anelliptical housing. Single-stage, rotary-vane units are used for vacuums to 28 inches Hg. Twostage units can draw vacuums to 29.7 inches Hg. The vacuum pump has an advantage over theair ejector in that it requires no source of steam for its operation. They are normally used as theinitial source of vacuum for condenser start-up.

Rev. 0 ME-02Page 17



Summary

The important information from this chapter is summarized below.

Heat Exchanger Applications Summary

Heat exchangers are often used in the following applications.

PreheaterRadiatorAir conditioning evaporator and condenserSteam condenser

The purpose of a condenser is to remove the latent heat of vaporization, condensingthe vapor into a liquid.

Heat exchangers condense the steam vapor into a liquid for return to the boiler.

The cycle's efficiency is increased by ensuring the maximum ∆T between the sourceand the heat sink.

The hotwell is the area at the bottom of the condenser where the condensed steamis collected to be pumped back into the system feedwater.

Condensate depression is the amount the condensate in a condenser is cooled belowsaturation (degrees subcooled).

Condensers operate at a vacuum to ensure the temperature (and thus the pressure)of the steam is as low as possible. This maximizes the ∆T and ∆P between thesource and the heat sink, ensuring the highest cycle efficiency possible.

ME-02 Rev. 0Page 18




Pumps


Pumps DOE-HDBK-1018/1-93 TABLE OF CONTENTS

TABLE OF CONTENTS


LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

CENTRIFUGAL PUMPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Diffuser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Impeller Classification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Centrifugal Pump Classification by Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Multi-Stage Centrifugal Pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Centrifugal Pump Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

CENTRIFUGAL PUMP OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Net Positive Suction Head. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Preventing Cavitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Centrifugal Pump Characteristic Curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Centrifugal Pump Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Gas Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Priming Centrifugal Pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

POSITIVE DISPLACEMENT PUMPS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Principle of Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Reciprocating Pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Rotary Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Diaphragm Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Positive Displacement Pump Characteristic Curves. . . . . . . . . . . . . . . . . . . . . 27Positive Displacement Pump Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Rev. 0 ME-03Page i


LIST OF FIGURES DOE-HDBK-1018/1-93 Pumps

LIST OF FIG URES

Figure 1 Centrifugal Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 2 Single and Double Volutes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 3 Centrifugal Pump Diffuser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 4 Single Suction and Double Suction Impellers. . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 5 Open, Semi-Open, and Enclosed Impellers. . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 6 Radial Flow Centrifugal Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 7 Axial Flow Centrifugal Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 8 Mixed Flow Centrifugal Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 9 Multi-Stage Centrifugal Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 10 Centrifugal Pump Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 11 Centrifugal Pump Characteristic Curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 12 Reciprocating Positive Displacement Pump Operation. . . . . . . . . . . . . . . . . . 19

Figure 13 Single-Acting and Double-Acting Pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 14 Simple Gear Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 15 Types of Gears Used In Pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 16 Lobe Type Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 17 Two-Screw, Low-Pitch, Screw Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 18 Three-Screw, High-Pitch, Screw Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 19 Rotary Moving Vane Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 20 Diaphragm Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 21 Positive Displacement Pump Characteristic Curve. . . . . . . . . . . . . . . . . . . . . 27

ME-03 Rev. 0Page ii


Pumps DOE-HDBK-1018/1-93 LIST OF TABLES

LIST OF TABLES

None

Rev. 0 ME-03Page iii


REFERENCES DOE-HDBK-1018/1-93 Pumps

REFERENCES

Babcock & Wilcox, Steam, Its Generations and Use, Babcock & Wilcox Co.


General Physics, Heat Transfer, Thermodynamics and Fluid Flow Fundamentals, GeneralPhysics Corporation.

Academic Program for Nuclear Power Plant Personnel, Volume III, Columbia, MD,General Physics Corporation, Library of Congress Card #A 326517, 1982.

Stewart, Harry L., Pneumatics & Hydraulics, Theodore Audel & Company.

ME-03 Rev. 0Page iv


Pumps DOE-HDBK-1018/1-93 OBJECTIVES

TERMINAL OBJECTIVE

1.0 Without references, DESCRIBE the purpose, construction, and principles of operation forcentrifugal pumps.

ENABLING OBJECTIVES

1.1 STATE the purposes of the following centrifugal pump components:

a. Impellerb. Volutec. Diffuser

d. Packinge. Lantern Ringf. Wearing ring

1.2 Given a drawing of a centrifugal pump, IDENTIFY the following majorcomponents:

a. Pump casingb. Pump shaftc. Impellerd. Volutee. Stuffing box

f. Stuffing box glandg. Packingh. Lantern Ringi. Impeller wearing ringj. Pump casing wearing ring

1.3 DEFINE the following terms:

a. Net Positive Suction Head Availableb. Cavitationc. Gas binding

d. Shutoff heade. Pump runout

1.4 STATE the relationship between net positive suction head available and net positivesuction head required that is necessary to avoid cavitation.

1.5 LIST three indications that a centrifugal pump may be cavitating.

1.6 LIST five changes that can be made in a pump or its surrounding system that can reducecavitation.

1.7 LIST three effects of cavitation.

1.8 DESCRIBE the shape of the characteristic curve for a centrifugal pump.

1.9 DESCRIBE how centrifugal pumps are protected from the conditions of dead headingand pump runout.

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OBJECTIVES DOE-HDBK-1018/1-93 Pumps

TERMINAL OBJECTIVE

2.0 Without references, DESCRIBE the purpose, construction, and principle of operation forpositive displacement pumps.

ENABLING OBJECTIVES

2.1 STATE the difference between the flow characteristics of centrifugal and positivedisplacement pumps.

2.2 Given a simplified drawing of a positive displacement pump, CLASSIFY the pump asone of the following:

a. Reciprocating piston pumpb. Gear-type rotary pumpc. Screw-type rotary pumpd. Lobe-type rotary pumpe. Moving vane pumpf. Diaphragm pump

2.3 EXPLAIN the importance of viscosity as it relates to the operation of a reciprocatingpositive displacement pump.

2.4 DESCRIBE the characteristic curve for a positive displacement pump.

2.5 DEFINE the term slippage.

2.6 STATE how positive displacement pumps are protected against overpressurization.

ME-03 Rev. 0Page vi


Pumps DOE-HDBK-1018/1-93 CENTRIFUGAL PUMPS

CENTRIFUGAL PUMPS

Centrifugal pumps are the most common type of pumps found in DOE facilities.Centrifugal pumps enjoy widespread application partly due to their ability tooperate over a wide range of flow rates and pump heads.

EO 1.1 STATE the purposes of the following centrifugal pumpcomponents:

a. Impellerb. Volutec. Diffuser

d. Packinge. Lantern Ringf. Wearing ring

EO 1.2 Given a drawing of a centrifugal pump, IDENTIFY thefollowing major components:

a. Pump casingb. Pump shaftc. Impellerd. Volutee. Stuffing box

f. Stuffing box glandg. Packingh. Lantern Ringi. Impeller wearing ringj. Pump casing wearing ring

I ntr oduction

Centrifugal pumps basically consist of a stationary pump casing and an impeller mounted on arotating shaft. The pump casing provides a pressure boundary for the pump and containschannels to properly direct the suction and discharge flow. The pump casing has suction anddischarge penetrations for the main flow path of the pump and normally has small drain and ventfittings to remove gases trapped in the pump casing or to drain the pump casing for maintenance.

Figure 1 is a simplified diagram of a typical centrifugal pump that shows the relative locationsof the pump suction, impeller, volute, and discharge. The pump casing guides the liquid fromthe suction connection to the center, or eye, of the impeller. The vanes of the rotating impellerimpart a radial and rotary motion to the liquid, forcing it to the outer periphery of the pumpcasing where it is collected in the outer part of the pump casing called the volute. The voluteis a region that expands in cross-sectional area as it wraps around the pump casing. The purposeof the volute is to collect the liquid discharged from the periphery of the impeller at highvelocity and gradually cause a reduction in fluid velocity by increasing the flow area. Thisconverts the velocity head to static pressure. The fluid is then discharged from the pumpthrough the discharge connection.

Rev. 0 ME-03Page 1


CENTRIFUGAL PUMPS DOE-HDBK-1018/1-93 Pumps

Figure 1 Centrifugal Pump

Centrifugal pumps can also be constructed in a manner that results in two distinct volutes, eachreceiving the liquid that is discharged from a 180o region of the impeller at any given time.Pumps of this type are called double volute pumps (they may also be referred to a split volutepumps). In some applications the double volute minimizes radial forces imparted to the shaft andbearings due to imbalances in the pressure around the impeller. A comparison of single anddouble volute centrifugal pumps is shown on Figure 2.

Figure 2 Single and Double Volutes

ME-03 Rev. 0Page 2



Diffuser

Figure 3 Centrifugal Pump Diffuser

Some centrifugal pumps containdiffusers. A diffuser is a set ofstationary vanes that surround theimpeller. The purpose of thediffuser is to increase theefficiency of the centrifugal pumpby allowing a more gradualexpansion and less turbulent areafor the liquid to reduce in velocity.The diffuser vanes are designed ina manner that the liquid exiting theimpeller will encounter an ever-increasing flow area as it passesthrough the diffuser. This increasein flow area causes a reduction inflow velocity, converting kineticenergy into flow pressure.

I mpeller Classif ication

Impellers of pumps are classified

Figure 4 Single-Suction and Double-Suction Impellers

based on the number of points thatthe liquid can enter the impellerand also on the amount ofwebbing between the impellerblades.

Impellers can be either single-suction or double-suction. Asingle-suction impeller allowsliquid to enter the center of theblades from only one direction. Adouble-suction impeller allowsliquid to enter the center of theimpeller blades from both sidessimultaneously. Figure 4 showssimplified diagrams of single anddouble-suction impellers.

Rev. 0 ME-03Page 3



Impellers can be open, semi-open, or enclosed. The open impeller consists only of bladesattached to a hub. The semi-open impeller is constructed with a circular plate (the web) attachedto one side of the blades. The enclosed impeller has circular plates attached to both sides of theblades. Enclosed impellers are also referred to as shrouded impellers. Figure 5 illustratesexamples of open, semi-open, and enclosed impellers.

Figure 5 Open, Semi-Open, and Enclosed Impellers

The impeller sometimes contains balancing holes that connect the space around the hub to thesuction side of the impeller. The balancing holes have a total cross-sectional area that isconsiderably greater than the cross-sectional area of the annular space between the wearing ringand the hub. The result is suction pressure on both sides of the impeller hub, which maintainsa hydraulic balance of axial thrust.

Centr ifugal Pump Classif ication by Flow

Centrifugal pumps can be classified based on the manner in which fluid flows through the pump.The manner in which fluid flows through the pump is determined by the design of the pumpcasing and the impeller. The three types of flow through a centrifugal pump are radial flow, axialflow, and mixed flow.

Radial Flow Pumps

In a radial flow pump, the liquid enters at the center of the impeller and is directed outalong the impeller blades in a direction at right angles to the pump shaft. The impellerof a typical radial flow pump and the flow through a radial flow pump are shown inFigure 6.

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Axial Flow Pumps

Figure 6 Radial Flow Centrifugal Pump

In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pumpshaft. Axial flow pumps are sometimes called propeller pumps because they operateessentially the same as the propeller of a boat. The impeller of a typical axial flow pumpand the flow through a radial flow pump are shown in Figure 7.

Figure 7 Axial Flow Centrifugal Pump

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Mix ed Flow Pumps

Mixed flow pumps borrow characteristics from both radial flow and axial flow pumps.As liquid flows through the impeller of a mixed flow pump, the impeller blades push theliquid out away from the pump shaft and to the pump suction at an angle greater than90o. The impeller of a typical mixed flow pump and the flow through a mixed flowpump are shown in Figure 8.

Figure 8 Mixed Flow Centrifugal Pump

Multi-Stage Centr ifugal Pumps

A centrifugal pump with a single impeller that can develop a differential pressure of more than150 psid between the suction and the discharge is difficult and costly to design and construct.A more economical approach to developing high pressures with a single centrifugal pump is toinclude multiple impellers on a common shaft within the same pump casing. Internal channelsin the pump casing route the discharge of one impeller to the suction of another impeller.Figure 9 shows a diagram of the arrangement of the impellers of a four-stage pump. The waterenters the pump from the top left and passes through each of the four impellers in series, goingfrom left to right. The water goes from the volute surrounding the discharge of one impeller tothe suction of the next impeller.

A pump stage is defined as that portion of a centrifugal pump consisting of one impeller and itsassociated components. Most centrifugal pumps are single-stage pumps, containing only oneimpeller. A pump containing seven impellers within a single casing would be referred to as aseven-stage pump or, or generally, as a multi-stage pump.

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Figure 9 Multi-Stage Centrifugal Pump

Centr ifugal Pump Components

Centrifugal pumps vary in design and construction from simple pumps with relatively few partsto extremely complicated pumps with hundreds of individual parts. Some of the most commoncomponents found in centrifugal pumps are wearing rings, stuffing boxes, packing, and lanternrings. These components are shown in Figure 10 and described on the following pages.

W earing Rings

Centrifugal pumps contain rotating impellers within stationary pump casings. To allowthe impeller to rotate freely within the pump casing, a small clearance is designed to bemaintained between the impeller and the pump casing. To maximize the efficiency of acentrifugal pump, it is necessary to minimize the amount of liquid leaking through thisclearance from the high pressure or discharge side of the pump back to the low pressureor suction side.

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Some wear or erosion will occur at the point where the impeller and the pump casing

Figure 10 Centrifugal Pump Components

nearly come into contact. This wear is due to the erosion caused by liquid leakingthrough this tight clearance and other causes. As wear occurs, the clearances becomelarger and the rate of leakage increases. Eventually, the leakage could becomeunacceptably large and maintenance would be required on the pump.

To minimize the cost of pump maintenance, many centrifugal pumps are designed withwearing rings. Wearing rings are replaceable rings that are attached to the impeller and/orthe pump casing to allow a small running clearance between the impeller and the pumpcasing without causing wear of the actual impeller or pump casing material. Thesewearing rings are designed to be replaced periodically during the life of a pump andprevent the more costly replacement of the impeller or the casing.

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Stuffing Box

In almost all centrifugal pumps, the rotating shaft that drives the impeller penetrates thepressure boundary of the pump casing. It is important that the pump is designed properlyto control the amount of liquid that leaks along the shaft at the point that the shaftpenetrates the pump casing. There are many different methods of sealing the shaftpenetration of the pump casing. Factors considered when choosing a method include thepressure and temperature of the fluid being pumped, the size of the pump, and thechemical and physical characteristics of the fluid being pumped.

One of the simplest types of shaft seal is the stuffing box. The stuffing box is acylindrical space in the pump casing surrounding the shaft. Rings of packing materialare placed in this space. Packing is material in the form of rings or strands that is placedin the stuffing box to form a seal to control the rate of leakage along the shaft. Thepacking rings are held in place by a gland. The gland is, in turn, held in place by studswith adjusting nuts. As the adjusting nuts are tightened, they move the gland in andcompress the packing. This axial compression causes the packing to expand radially,forming a tight seal between the rotating shaft and the inside wall of the stuffing box.

The high speed rotation of the shaft generates a significant amount of heat as it rubsagainst the packing rings. If no lubrication and cooling are provided to the packing, thetemperature of the packing increases to the point where damage occurs to the packing,the pump shaft, and possibly nearby pump bearings. Stuffing boxes are normallydesigned to allow a small amount of controlled leakage along the shaft to providelubrication and cooling to the packing. The leakage rate can be adjusted by tighteningand loosening the packing gland.

L anter n Ring

It is not always possible to use a standard stuffing box to seal the shaft of a centrifugalpump. The pump suction may be under a vacuum so that outward leakage is impossibleor the fluid may be too hot to provide adequate cooling of the packing. These conditionsrequire a modification to the standard stuffing box.

One method of adequately cooling the packing under these conditions is to include alantern ring. A lantern ring is a perforated hollow ring located near the center of thepacking box that receives relatively cool, clean liquid from either the discharge of thepump or from an external source and distributes the liquid uniformly around the shaft toprovide lubrication and cooling. The fluid entering the lantern ring can cool the shaft andpacking, lubricate the packing, or seal the joint between the shaft and packing againstleakage of air into the pump in the event the pump suction pressure is less than that ofthe atmosphere.

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Mechanical Seals

In some situations, packing material is not adequate for sealing the shaft. One commonalternative method for sealing the shaft is with mechanical seals. Mechanical sealsconsist of two basic parts, a rotating element attached to the pump shaft and a stationaryelement attached to the pump casing. Each of these elements has a highly polishedsealing surface. The polished faces of the rotating and stationary elements come intocontact with each other to form a seal that prevents leakage along the shaft.

Summary


Centrifugal Pumps Summary

The impeller contains rotating vanes that impart a radial and rotary motion to theliquid.

The volute collects the liquid discharged from the impeller at high velocity andgradually causes a reduction in fluid velocity by increasing the flow area, convertingthe velocity head to a static head.

A diffuser increases the efficiency of a centrifugal pump by allowing a more gradualexpansion and less turbulent area for the liquid to slow as the flow area expands.

Packing material provides a seal in the area where the pump shaft penetrates thepump casing.

Wearing rings are replaceable rings that are attached to the impeller and/or thepump casing to allow a small running clearance between the impeller and pumpcasing without causing wear of the actual impeller or pump casing material.

The lantern ring is inserted between rings of packing in the stuffing box to receiverelatively cool, clean liquid and distribute the liquid uniformly around the shaft toprovide lubrication and cooling to the packing.

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Pumps DOE-HDBK-1018/1-93 CENTRIFUGAL PUMP OPERATION

CENTRIFUGAL PUMP OPERATION

Improper operation of centrifugal pumps can result in damage to the pump andloss of function of the system that the pump is installed in. It is helpful to knowwhat conditions can lead to pump damage to allow better understanding of pumpoperating procedures and how the procedures aid the operator in avoiding pumpdamage.

EO 1.3 DEFINE the following terms:

a. Net Positive SuctionHead Available

b. Cavitation

c. Gas bindingd. Shutoff heade. Pump runout

EO 1.4 STATE the relationship between net positive suction headavailable and net positive suction head required that isnecessary to avoid cavitation.

EO 1.5 LIST three indications that a centrifugal pump may becavitating.

EO 1.6 LIST five changes that can be made in a pump or itssurrounding system that can reduce cavitation.

EO 1.7 LIST three effects of cavitation.

EO 1.8 DESCRIBE the shape of the characteristic curve for acentrifugal pump.

EO 1.9 DESCRIBE how centrifugal pumps are protected fromthe conditions of dead heading and pump runout.

I ntr oduction

Many centrifugal pumps are designed in a manner that allows the pump to operate continuouslyfor months or even years. These centrifugal pumps often rely on the liquid that they arepumping to provide cooling and lubrication to the pump bearings and other internal componentsof the pump. If flow through the pump is stopped while the pump is still operating, the pumpwill no longer be adequately cooled and the pump can quickly become damaged. Pump damagecan also result from pumping a liquid whose temperature is close to saturated conditions.

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CENTRIFUGAL PUMP OPERATION DOE-HDBK-1018/1 Pumps

Cavitation

The flow area at the eye of the pump impeller is usually smaller than either the flow area of thepump suction piping or the flow area through the impeller vanes. When the liquid being pumpedenters the eye of a centrifugal pump, the decrease in flow area results in an increase in flowvelocity accompanied by a decrease in pressure. The greater the pump flow rate, the greater thepressure drop between the pump suction and the eye of the impeller. If the pressure drop islarge enough, or if the temperature is high enough, the pressure drop may be sufficient to causethe liquid to flash to vapor when the local pressure falls below the saturation pressure for thefluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impellerare swept along the impeller vanes by the flow of the fluid. When the bubbles enter a regionwhere local pressure is greater than saturation pressure farther out the impeller vane, the vaporbubbles abruptly collapse. This process of the formation and subsequent collapse of vaporbubbles in a pump is called cavitation.

Cavitation in a centrifugal pump has a significant effect on pump performance. Cavitationdegrades the performance of a pump, resulting in a fluctuating flow rate and discharge pressure.Cavitation can also be destructive to pumps internal components. When a pump cavitates, vaporbubbles form in the low pressure region directly behind the rotating impeller vanes. These vaporbubbles then move toward the oncoming impeller vane, where they collapse and cause a physicalshock to the leading edge of the impeller vane. This physical shock creates small pits on theleading edge of the impeller vane. Each individual pit is microscopic in size, but the cumulativeeffect of millions of these pits formed over a period of hours or days can literally destroy a pumpimpeller. Cavitation can also cause excessive pump vibration, which could damage pumpbearings, wearing rings, and seals.

A small number of centrifugal pumps are designed to operate under conditions where cavitationis unavoidable. These pumps must be specially designed and maintained to withstand the smallamount of cavitation that occurs during their operation. Most centrifugal pumps are not designedto withstand sustained cavitation.

Noise is one of the indications that a centrifugal pump is cavitating. A cavitating pump cansound like a can of marbles being shaken. Other indications that can be observed from a remoteoperating station are fluctuating discharge pressure, flow rate, and pump motor current. Methodsto stop or prevent cavitation are presented in the following paragraphs.

Net Positive Suction Head

To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pumpmust remain above saturation pressure. The quantity used to determine if the pressure of theliquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH).The net positive suction head available (NPSHA) is the difference between the pressure at thesuction of the pump and the saturation pressure for the liquid being pumped. The net positivesuction head required (NPSHR) is the minimum net positive suction head necessary to avoidcavitation.

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The condition that must exist to avoid cavitation is that the net positive suction head availablemust be greater than or equal to the net positive suction head required. This requirement can bestated mathematically as shown below.

NPSHA ≥ NPSHR

A formula for NPSHA can be stated as the following equation.

NPSHA = Psuction - Psaturation

When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at thesuction of the pump is the sum of the absolute pressure at the surface of the liquid in the tankplus the pressure due to the elevation difference between the surface of liquid in the tank andthe pump suction less the head losses due to friction in the suction line from the tank to thepump.

NPSHA = Pa + Pst - hf - Psat

Where:

NPSHA = net positive suction head availablePa = absolute pressure on the surface of the liquidPst = pressure due to elevation between liquid surface and pump suctionhf = head losses in the pump suction pipingPsat = saturation pressure of the liquid being pumped

Pr eventing Cavitation

If a centrifugal pump is cavitating, several changes in the system design or operation may benecessary to increase the NPSHA above the NPSHR and stop the cavitation. One method forincreasing the NPSHA is to increase the pressure at the suction of the pump. For example, if apump is taking suction from an enclosed tank, either raising the level of the liquid in the tank orincreasing the pressure in the space above the liquid increases suction pressure.

It is also possible to increase the NPSHA by decreasing the temperature of the liquid beingpumped. Decreasing the temperature of the liquid decreases the saturation pressure, causingNPSHA to increase. Recall from the previous module on heat exchangers that large steamcondensers usually subcool the condensate to less than the saturation temperature, calledcondensate depression, to prevent cavitation in the condensate pumps.

If the head losses in the pump suction piping can be reduced, the NPSHA will be increased.Various methods for reducing head losses include increasing the pipe diameter, reducing thenumber of elbows, valves, and fittings in the pipe, and decreasing the length of the pipe.

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It may also be possible to stop cavitation by reducing the NPSHR for the pump. The NPSHR isnot a constant for a given pump under all conditions, but depends on certain factors. Typically,the NPSHR of a pump increases significantly as flow rate through the pump increases.Therefore, reducing the flow rate through a pump by throttling a discharge valve decreasesNPSHR. NPSHR is also dependent upon pump speed. The faster the impeller of a pump rotates,the greater the NPSHR. Therefore, if the speed of a variable speed centrifugal pump is reduced,the NPSHR of the pump decreases. However, since a pump's flow rate is most often dictatedby the needs of the system on which it is connected, only limited adjustments can be madewithout starting additional parallel pumps, if available.

The net positive suction head required to prevent cavitation is determined through testing by thepump manufacturer and depends upon factors including type of impeller inlet, impeller design,pump flow rate, impeller rotational speed, and the type of liquid being pumped. Themanufacturer typically supplies curves of NPSHR as a function of pump flow rate for a particularliquid (usually water) in the vendor manual for the pump.

Centr ifugal Pump Character istic Cur ves

For a given centrifugal pump operating at a constant speed, the flow rate through the pump is

Figure 11 Centrifugal Pump Characteristic Curve

dependent upon the differential pressure or head developed by the pump. The lower the pumphead, the higher the flow rate. A vendor manual for a specific pump usually contains a curveof pump flow rate versus pump head called a pump characteristic curve. After a pump isinstalled in a system, it is usually tested to ensure that the flow rate and head of the pump arewithin the required specifications. A typical centrifugal pump characteristic curve is shown inFigure 11.

There are several terms associated with the pump characteristic curve that must be defined.Shutoff head is the maximum head that can be developed by a centrifugal pump operating at aset speed. Pump runout is the maximum flow that can be developed by a centrifugal pumpwithout damaging the pump. Centrifugal pumps must be designed and operated to be protectedfrom the conditions of pump runout or operating at shutoff head. Additional information maybe found in the handbook on Thermodynamics, Heat Transfer, and Fluid Flow.

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Centr ifugal Pump Pr otection

A centrifugal pump is dead-headed when it is operated with no flow through it, for example, witha closed discharge valve or against a seated check valve. If the discharge valve is closed andthere is no other flow path available to the pump, the impeller will churn the same volume ofwater as it rotates in the pump casing. This will increase the temperature of the liquid (due tofriction) in the pump casing to the point that it will flash to vapor. The vapor can interrupt thecooling flow to the pump's packing and bearings, causing excessive wear and heat. If the pumpis run in this condition for a significant amount of time, it will become damaged.

When a centrifugal pump is installed in a system such that it may be subjected to periodic shutoffhead conditions, it is necessary to provide some means of pump protection. One method forprotecting the pump from running dead-headed is to provide a recirculation line from the pumpdischarge line upstream of the discharge valve, back to the pump's supply source. Therecirculation line should be sized to allow enough flow through the pump to prevent overheatingand damage to the pump. Protection may also be accomplished by use of an automatic flowcontrol device.

Centrifugal pumps must also be protected from runout. Runout can lead to cavitation and canalso cause overheating of the pump's motor due to excessive currents. One method for ensuringthat there is always adequate flow resistance at the pump discharge to prevent excessive flowthrough the pump is to place an orifice or a throttle valve immediately downstream of the pumpdischarge. Properly designed piping systems are very important to protect from runout.

Gas Binding

Gas binding of a centrifugal pump is a condition where the pump casing is filled with gases orvapors to the point where the impeller is no longer able to contact enough fluid to functioncorrectly. The impeller spins in the gas bubble, but is unable to force liquid through the pump.This can lead to cooling problems for the pump's packing and bearings.

Centrifugal pumps are designed so that their pump casings are completely filled with liquidduring pump operation. Most centrifugal pumps can still operate when a small amount of gasaccumulates in the pump casing, but pumps in systems containing dissolved gases that are notdesigned to be self-venting should be periodically vented manually to ensure that gases do notbuild up in the pump casing.

Pr iming Centr ifugal Pumps

Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled withliquid before the pump is started, or the pump will not be able to function. If the pump casingbecomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable ofpumping. To ensure that a centrifugal pump remains primed and does not become gas-bound,most centrifugal pumps are located below the level of the source from which the pump is to takeits suction. The same effect can be gained by supplying liquid to the pump suction underpressure supplied by another pump placed in the suction line.

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Summary


Centrifugal Pump Operation Summary

There are three indications that a centrifugal pump is cavitating.NoiseFluctuating discharge pressure and flowFluctuating pump motor current

Steps that can be taken to stop pump cavitation include:Increase the pressure at the suction of the pump.Reduce the temperature of the liquid being pumped.Reduce head losses in the pump suction piping.Reduce the flow rate through the pump.Reduce the speed of the pump impeller.

Three effects of pump cavitation are:Degraded pump performanceExcessive pump vibrationDamage to pump impeller, bearings, wearing rings, and seals

To avoid pump cavitation, the net positive suction head available must be greaterthan the net positive suction head required.

Net positive suction head available is the difference between the pump suctionpressure and the saturation pressure for the liquid being pumped.

Cavitation is the process of the formation and subsequent collapse of vapor bubblesin a pump.

Gas binding of a centrifugal pump is a condition where the pump casing is filledwith gases or vapors to the point where the impeller is no longer able to contactenough fluid to function correctly.

Shutoff head is the maximum head that can be developed by a centrifugal pumpoperating at a set speed.

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Centrifugal Pump Operation Summary (Cont.)

Pump runout is the maximum flow that can be developed by a centrifugal pumpwithout damaging the pump.

The greater the head against which a centrifugal pump operates, the lower the flowrate through the pump. The relationship between pump flow rate and head isillustrated by the characteristic curve for the pump.

Centrifugal pumps are protected from dead-heading by providing a recirculationfrom the pump discharge back to the supply source of the pump.

Centrifugal pumps are protected from runout by placing an orifice or throttle valveimmediately downstream of the pump discharge and through proper piping systemdesign.

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POSITIVE DISPLACEMENT PUMPS DOE-HDBK-1018/1-93 Pumps

POSITIVE DISPLACEMENT PUMPS

Positive displacement pumps operate on a different principle than centrifugalpumps. Positive displacement pumps physically entrap a quantity of liquid at thesuction of the pump and push that quantity out the discharge of the pump.

EO 2.1 STATE the difference between the flow characteristics ofcentrifugal and positive displacement pumps.

EO 2.2 Given a simplified drawing of a positive displacement pump,CLASSIFY the pump as one of the following:

a. Reciprocating piston pumpb. Gear-type rotary pumpc. Screw-type rotary pumpd. Lobe-type rotary pump

e. Moving vane pumpf. Diaphragm pump

EO 2.3 EXPLAIN the importance of viscosity as it relates to theoperation of a reciprocating positive displacement pump.

EO 2.4 DESCRIBE the characteristic curve for a positivedisplacement pump.

EO 2.5 DEFINE the term slippage.

EO 2.6 STATE how positive displacement pumps are protectedagainst overpressurization.

I ntr oduction

A positive displacement pump is one in which a definite volume of liquid is delivered for eachcycle of pump operation. This volume is constant regardless of the resistance to flow offeredby the system the pump is in, provided the capacity of the power unit driving the pump or pumpcomponent strength limits are not exceeded. The positive displacement pump delivers liquid inseparate volumes with no delivery in between, although a pump having several chambers mayhave an overlapping delivery among individual chambers, which minimizes this effect. Thepositive displacement pump differs from centrifugal pumps, which deliver a continuous flow forany given pump speed and discharge resistance.

Positive displacement pumps can be grouped into three basic categories based on their designand operation. The three groups are reciprocating pumps, rotary pumps, and diaphragm pumps.

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Pumps DOE-HDBK-1018/1-93 POSITIVE DISPLACEMENT PUMPS

Pr inciple of Oper ation

All positive displacement pumps operate on the same basic principle. This principle can be mosteasily demonstrated by considering a reciprocating positive displacement pump consisting of asingle reciprocating piston in a cylinder with a single suction port and a single discharge port asshown in Figure 12. Check valves in the suction and discharge ports allow flow in only onedirection.

During the suction stroke, the piston moves to the left, causing the check valve in the suction

Figure 12 Reciprocating Positive Displacement Pump Operation

line between the reservoir and the pump cylinder to open and admit water from the reservoir.During the discharge stroke, the piston moves to the right, seating the check valve in the suctionline and opening the check valve in the discharge line. The volume of liquid moved by thepump in one cycle (one suction stroke and one discharge stroke) is equal to the change in theliquid volume of the cylinder as the piston moves from its farthest left position to its farthestright position.

Recipr ocating Pumps

Reciprocating positive displacement pumps are generally categorized in four ways: direct-actingor indirect-acting; simplex or duplex; single-acting or double-acting; and power pumps.

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Dir ect-Acting and I ndir ect-Acting Pumps

Some reciprocating pumps are powered by prime movers that also have reciprocatingmotion, such as a reciprocating pump powered by a reciprocating steam piston. The pistonrod of the steam piston may be directly connected to the liquid piston of the pump or it maybe indirectly connected with a beam or linkage. Direct-acting pumps have a plunger on theliquid (pump) end that is directly driven by the pump rod (also the piston rod or extensionthereof) and carries the piston of the power end. Indirect-acting pumps are driven by meansof a beam or linkage connected to and actuated by the power piston rod of a separatereciprocating engine.

Simplex and Duplex Pumps

A simplex pump, sometimes referred to as a single pump, is a pump having a single liquid(pump) cylinder. A duplex pump is the equivalent of two simplex pumps placed side byside on the same foundation.

The driving of the pistons of a duplex pump is arranged in such a manner that when onepiston is on its upstroke the other piston is on its downstroke, and vice versa. Thisarrangement doubles the capacity of the duplex pump compared to a simplex pump ofcomparable design.

Single-Acting and Double-Acting Pumps

A single-acting pump is one that takes a suction, filling the pump cylinder on the stroke inonly one direction, called the suction stroke, and then forces the liquid out of the cylinderon the return stroke, called the discharge stroke. A double-acting pump is one that, as itfills one end of the liquid cylinder, is discharging liquid from the other end of the cylinder.On the return stroke, the end of the cylinder just emptied is filled, and the end just filledis emptied. One possible arrangement for single-acting and double-acting pumps is shownin Figure 13.

Power Pumps

Power pumps convert rotary motion to low speed reciprocating motion by reductiongearing, a crankshaft, connecting rods and crossheads. Plungers or pistons are driven bythe crosshead drives. Rod and piston construction, similar to duplex double-acting steampumps, is used by the liquid ends of the low pressure, higher capacity units. The higherpressure units are normally single-acting plungers, and usually employ three (triplex)plungers. Three or more plungers substantially reduce flow pulsations relative to simplexand even duplex pumps.

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Power pumps typically have high efficiency and are capable of developing very high pressures.

Figure 13 Single-Acting and Double-Acting Pumps

They can be driven by either electric motors or turbines. They are relatively expensive pumpsand can rarely be justified on the basis of efficiency over centrifugal pumps. However, they arefrequently justified over steam reciprocating pumps where continuous duty service is needed dueto the high steam requirements of direct-acting steam pumps.

In general, the effective flow rate of reciprocating pumps decreases as the viscosity of the fluidbeing pumped increases because the speed of the pump must be reduced. In contrast tocentrifugal pumps, the differential pressure generated by reciprocating pumps is independent offluid density. It is dependent entirely on the amount of force exerted on the piston. For moreinformation on viscosity, density, and positive displacement pump theory, refer to the handbookon Thermodynamics, Heat Transfer, and Fluid Flow.

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Rotary Pumps

Rotary pumps operate on the principle that a rotating vane, screw, or gear traps the liquid in thesuction side of the pump casing and forces it to the discharge side of the casing. These pumpsare essentially self-priming due to their capability of removing air from suction lines andproducing a high suction lift. In pumps designed for systems requiring high suction lift and self-priming features, it is essential that all clearances between rotating parts, and between rotatingand stationary parts, be kept to a minimum in order to reduce slippage. Slippage is leakage offluid from the discharge of the pump back to its suction.

Due to the close clearances in rotary pumps, it is necessary to operate these pumps at relativelylow speed in order to secure reliable operation and maintain pump capacity over an extendedperiod of time. Otherwise, the erosive action due to the high velocities of the liquid passingthrough the narrow clearance spaces would soon cause excessive wear and increased clearances,resulting in slippage.

There are many types of positive displacement rotary pumps, and they are normally grouped intothree basic categories that include gear pumps, screw pumps, and moving vane pumps.

Simple Gear Pump

There are several variations of

Figure 14 Simple Gear Pump

gear pumps. The simple gearpump shown in Figure 14consists of two spur gearsmeshing together and revolving inopposite directions within acasing. Only a few thousandthsof an inch clearance existsbetween the case and the gearfaces and teeth extremities. Anyliquid that fills the space boundedby two successive gear teeth andthe case must follow along withthe teeth as they revolve. Whenthe gear teeth mesh with the teethof the other gear, the spacebetween the teeth is reduced, andthe entrapped liquid is forced outthe pump discharge pipe. As thegears revolve and the teeth disengage, the space again opens on the suction side of thepump, trapping new quantities of liquid and carrying it around the pump case to thedischarge. As liquid is carried away from the suction side, a lower pressure is created,which draws liquid in through the suction line.

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With the large number of teeth usually employed on the gears, the discharge is relativelysmooth and continuous, with small quantities of liquid being delivered to the discharge linein rapid succession. If designed with fewer teeth, the space between the teeth is greater andthe capacity increases for a given speed; however, the tendency toward a pulsatingdischarge increases. In all simple gear pumps, power is applied to the shaft of one of thegears, which transmits power to the driven gear through their meshing teeth.

There are no valves in the gear pump to cause friction losses as in the reciprocating pump.The high impeller velocities, with resultant friction losses, are not required as in thecentrifugal pump. Therefore, the gear pump is well suited for handling viscous fluids suchas fuel and lubricating oils.

Other Gear Pumps

There are two types of gears used in gear pumps

Figure 15 Types of Gears Used In Pumps

in addition to the simple spur gear. One type isthe helical gear. A helix is the curve producedwhen a straight line moves up or down thesurface of a cylinder. The other type is theherringbone gear. A herringbone gear iscomposed of two helixes spiraling in differentdirections from the center of the gear. Spur,helical, and herringbone gears are shown inFigure 15.

The helical gear pump has advantages over thesimple spur gear. In a spur gear, the entirelength of the gear tooth engages at the sametime. In a helical gear, the point of engagementmoves along the length of the gear tooth as thegear rotates. This makes the helical gear operatewith a steadier discharge pressure and fewerpulsations than a spur gear pump.

The herringbone gear pump is also amodification of the simple gear pump. Itsprincipal difference in operation from the simplespur gear pump is that the pointed center sectionof the space between two teeth beginsdischarging before the divergent outer ends ofthe preceding space complete discharging. Thisoverlapping tends to provide a steadier dischargepressure. The power transmission from thedriving to the driven gear is also smoother andquieter.

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L obe Type Pump

Figure 16 Lobe Type Pump

The lobe type pump shown in Figure 16is another variation of the simple gearpump. It is considered as a simple gearpump having only two or three teeth perrotor; otherwise, its operation or theexplanation of the function of its parts isno different. Some designs of lobepumps are fitted with replaceable gibs,that is, thin plates carried in grooves atthe extremity of each lobe where theymake contact with the casing. The gibpromotes tightness and absorbs radialwear.

Scr ew-Type Positive Displacement Rotary Pump

There are many variations in the design of the screw type positive displacement, rotarypump. The primary differences consist of the number of intermeshing screws involved,the pitch of the screws, and the general direction of fluid flow. Two common designs arethe two-screw, low-pitch, double-flow pump and the three-screw, high-pitch, double-flowpump.

Two-Scr ew, L ow-Pitch, Scr ew Pump

The two-screw, low-pitch, screw pump consists of two screws that mesh with closeclearances, mounted on two parallel shafts. One screw has a right-handed thread, andthe other screw has a left-handed thread. One shaft is the driving shaft and drives theother shaft through a set of herringbone timing gears. The gears serve to maintainclearances between the screws as they turn and to promote quiet operation. Thescrews rotate in closely fitting duplex cylinders that have overlapping bores. Allclearances are small, but there is no actual contact between the two screws or betweenthe screws and the cylinder walls.

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The complete assembly and the usual flow

Figure 17 Two-Screw, Low-Pitch, Screw Pump

Figure 18 Three-Screw, High-Pitch, Screw Pump

path are shown in Figure 17. Liquid istrapped at the outer end of each pair ofscrews. As the first space between the screwthreads rotates away from the opposite screw,a one-turn, spiral-shaped quantity of liquid isenclosed when the end of the screw againmeshes with the opposite screw. As thescrew continues to rotate, the entrapped spiralturns of liquid slide along the cylinder towardthe center discharge space while the next slugis being entrapped. Each screw functionssimilarly, and each pair of screws dischargesan equal quantity of liquid in opposed streamstoward the center, thus eliminating hydraulicthrust. The removal of liquid from thesuction end by the screws produces areduction in pressure, which draws liquidthrough the suction line.

Thr ee-Scr ew, H igh-Pitch, Scr ew Pump

The three-screw, high-pitch, screw pump,shown in Figure 18, has many of the sameelements as the two-screw, low-pitch, screwpump, and their operations are similar.Three screws, oppositely threaded on eachend, are employed. They rotate in a triplecylinder, the two outer bores of whichoverlap the center bore. The pitch of thescrews is much higher than in the low pitchscrew pump; therefore, the center screw, orpower rotor, is used to drive the two outeridler rotors directly without external timinggears. Pedestal bearings at the base supportthe weight of the rotors and maintain theiraxial position. The liquid being pumpedenters the suction opening, flows throughpassages around the rotor housing, andthrough the screws from each end, in opposedstreams, toward the center discharge. Thiseliminates unbalanced hydraulic thrust. Thescrew pump is used for pumping viscousfluids, usually lubricating, hydraulic, or fueloil.

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Rotary Moving Vane Pump

The rotary moving vane pump shown in Figure 19 is another type of positive displacementpump used. The pump consists of a cylindrically bored housing with a suction inlet on oneside and a discharge outlet on the other. A cylindrically shaped rotor with a diametersmaller than the cylinder is driven about an axis placed above the centerline of the cylinder.The clearance between rotor and cylinder is small at the top but increases at the bottom.The rotor carries vanes that move in and out as it rotates to maintain sealed spaces betweenthe rotor and the cylinder wall. The vanes trap liquid or gas on the suction side and carryit to the discharge side, where contraction of the space expels it through the discharge line.The vanes may swing on pivots, or they may slide in slots in the rotor.

Figure 19 Rotary Moving Vane Pump

Diaphr agm Pumps

Diaphragm pumps are also classified as positive displacement pumps because the diaphragm actsas a limited displacement piston. The pump will function when a diaphragm is forced intoreciprocating motion by mechanical linkage, compressed air, or fluid from a pulsating, externalsource. The pump construction eliminates any contact between the liquid being pumped and thesource of energy. This eliminates the possibility of leakage, which is important when handlingtoxic or very expensive liquids. Disadvantages include limited head and capacity range, and thenecessity of check valves in the suction and discharge nozzles. An example of a diaphragmpump is shown in Figure 20.

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Figure 20 Diaphragm Pump

Positive Displacement Pump Character istic Cur ves

Positive displacement pumps deliver a definite volume of

Figure 21 Positive Displacement Pump

Characteristic Curve

liquid for each cycle of pump operation. Therefore, theonly factor that effects flow rate in an ideal positivedisplacement pump is the speed at which it operates. Theflow resistance of the system in which the pump isoperating will not effect the flow rate through the pump.Figure 21 shows the characteristic curve for a positivedisplacement pump.

The dashed line in Figure 21 shows actual positivedisplacement pump performance. This line reflects thefact that as the discharge pressure of the pump increases,some amount of liquid will leak from the discharge of thepump back to the pump suction, reducing the effectiveflow rate of the pump. The rate at which liquid leaksfrom the pump discharge to its suction is called slippage.

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Positive Displacement Pump Pr otection

Positive displacement pumps are normally fitted with relief valves on the upstream side of theirdischarge valves to protect the pump and its discharge piping from overpressurization. Positivedisplacement pumps will discharge at the pressure required by the system they are supplying.The relief valve prevents system and pump damage if the pump discharge valve is shut duringpump operation or if any other occurrence such as a clogged strainer blocks system flow.

Summary


Positive Displacement Pumps Summary

The flow delivered by a centrifugal pump during one revolution of the impeller dependsupon the head against which the pump is operating. The positive displacementpump delivers a definite volume of fluid for each cycle of pump operationregardless of the head against which the pump is operating.

Positive displacement pumps may be classified in the following ways:Reciprocating piston pumpGear-type rotary pumpLobe-type rotary pumpScrew-type rotary pumpMoving vane pumpDiaphragm pump

As the viscosity of a liquid increases, the maximum speed at which a reciprocatingpositive displacement pump can properly operate decreases. Therefore, as viscosityincreases, the maximum flow rate through the pump decreases.

The characteristic curve for a positive displacement pump operating at a certainspeed is a vertical line on a graph of head versus flow.

Slippage is the rate at which liquid leaks from the discharge of the pump back tothe pump suction.

Positive displacement pumps are protected from overpressurization by a relief valveon the upstream side of the pump discharge valve.

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